Start plumbing through debug info type information with function parameters/return value (#6410)

This adds just enough debug info for i32/int parameters and return
values, with a path forward for adding DWARF type metadata for other
types.

As it happens, return type information is carried separately from
parameter information:
* Return type information is carried in the `type` of the `DISubprogram`
  (as a `DISubroutineType` - which does carry parameter type information
  as well, but that's unused when the DWARF is emitted by LLVM)
* Parameter information is carried by `DILocalVariable`s with a non-zero
  `arg` value (representing the order of function parameters)

In the absence of locations for the parameters (future work), nothing
would usually keep the `DILocalVariable` live/reachable when emitting
DWARF - so for cases where this can happen (for clang, this happens in
optimized builds where all references to the parameter variable might be
optimized away) the variables can be "retained" in a list on the
`DISubprogram` - achieved by passing `AlwaysPreserve` parameter to
`createParameterVariable` (adds them to a list, then that list gets
attached to the `DISubprogram` when it's finalized later)

For now, any unsupported types are emitted as `void*` (except void
return, which is implemented as void) as a placeholder.

Given this example:
```
import Core library "io";
class MyClass {
}
fn Unsupported(v: MyClass) {
}
fn Ret() -> i32 {
  return 42;
}
fn Arg(x: i32) {
  Core.Print(x);
}
fn Run() {
}
```
this is the resulting DWARF:
```
DW_TAG_compile_unit
  DW_AT_name    ("test.carbon")
  DW_TAG_subprogram
    DW_AT_name  ("Unsupported")
    DW_TAG_formal_parameter
      DW_AT_type        (0x00000066 "void *")
  DW_TAG_subprogram
    DW_AT_name  ("Ret")
    DW_AT_type  (0x00000062 "int")
  DW_TAG_subprogram
    DW_AT_name  ("Arg")
    DW_TAG_formal_parameter
      DW_AT_type        (0x00000062 "int")
  DW_TAG_subprogram
    DW_AT_name  ("Run")
  DW_TAG_base_type
    DW_AT_name  ("int")
  DW_TAG_pointer_type
```
And the debugger:
```
(gdb) p Ret()
$1 = 42
(gdb) p Arg(4)
4
$2 = void
```

I'm not sure if there's a way this logic should be merged with the logic
for making the `llvm::Function` type (which the `DISubroutineType`
building code was inspired by/copied from) - since they're done at
different times/places, I don't think there's an easy way to do it in
one pass, but maybe the code can be shared (even if it's run twice) in
some generic `SemIR::Function` type walker.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>

toolchain/lower/testdata/interop/cpp/enum.carbon

@@ -157,7 +157,7 @@ fn Call() {
 // CHECK:STDOUT: !6 = !DIFile(filename: "pass_as_arg.carbon", directory: "")
 // CHECK:STDOUT: !7 = distinct !DISubprogram(name: "Pass", linkageName: "_CPass.Main", scope: null, file: !6, line: 6, type: !8, spFlags: DISPFlagDefinition, unit: !5)
 // CHECK:STDOUT: !8 = !DISubroutineType(types: !9)
-// CHECK:STDOUT: !9 = !{}
+// CHECK:STDOUT: !9 = !{null}
 // CHECK:STDOUT: !10 = !DILocation(line: 7, column: 11, scope: !7)
 // CHECK:STDOUT: !11 = !DILocation(line: 8, column: 11, scope: !7)
 // CHECK:STDOUT: !12 = !DILocation(line: 9, column: 11, scope: !7)
@@ -190,18 +190,18 @@ fn Call() {
 // CHECK:STDOUT: ; Function Attrs: nounwind
 // CHECK:STDOUT: define void @_CConvertEnumToI16.Main(i16 %e) #0 !dbg !12 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @_CTakeI16.Main(i16 %e), !dbg !13
-// CHECK:STDOUT:   ret void, !dbg !14
+// CHECK:STDOUT:   call void @_CTakeI16.Main(i16 %e), !dbg !18
+// CHECK:STDOUT:   ret void, !dbg !19
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nounwind
-// CHECK:STDOUT: define void @_CConvertI16ToEnum.Main(i16 %n) #0 !dbg !15 {
+// CHECK:STDOUT: define void @_CConvertI16ToEnum.Main(i16 %n) #0 !dbg !20 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %.loc17_13.3.temp = alloca i16, align 2, !dbg !16
-// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc17_13.3.temp), !dbg !16
-// CHECK:STDOUT:   store i16 %n, ptr %.loc17_13.3.temp, align 2, !dbg !16
-// CHECK:STDOUT:   call void @_ZN1C1FENS_1EE.carbon_thunk(ptr %.loc17_13.3.temp), !dbg !17
-// CHECK:STDOUT:   ret void, !dbg !18
+// CHECK:STDOUT:   %.loc17_13.3.temp = alloca i16, align 2, !dbg !23
+// CHECK:STDOUT:   call void @llvm.lifetime.start.p0(ptr %.loc17_13.3.temp), !dbg !23
+// CHECK:STDOUT:   store i16 %n, ptr %.loc17_13.3.temp, align 2, !dbg !23
+// CHECK:STDOUT:   call void @_ZN1C1FENS_1EE.carbon_thunk(ptr %.loc17_13.3.temp), !dbg !24
+// CHECK:STDOUT:   ret void, !dbg !25
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nocallback nofree nosync nounwind willreturn memory(argmem: readwrite)
@@ -237,16 +237,23 @@ fn Call() {
 // CHECK:STDOUT: !6 = !DIFile(filename: "convert.carbon", directory: "")
 // CHECK:STDOUT: !7 = distinct !DISubprogram(name: "PassEnum", linkageName: "_CPassEnum.Main", scope: null, file: !6, line: 8, type: !8, spFlags: DISPFlagDefinition, unit: !5)
 // CHECK:STDOUT: !8 = !DISubroutineType(types: !9)
-// CHECK:STDOUT: !9 = !{}
+// CHECK:STDOUT: !9 = !{null}
 // CHECK:STDOUT: !10 = !DILocation(line: 9, column: 3, scope: !7)
 // CHECK:STDOUT: !11 = !DILocation(line: 8, column: 1, scope: !7)
-// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "ConvertEnumToI16", linkageName: "_CConvertEnumToI16.Main", scope: null, file: !6, line: 12, type: !8, spFlags: DISPFlagDefinition, unit: !5)
-// CHECK:STDOUT: !13 = !DILocation(line: 13, column: 3, scope: !12)
-// CHECK:STDOUT: !14 = !DILocation(line: 12, column: 1, scope: !12)
-// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "ConvertI16ToEnum", linkageName: "_CConvertI16ToEnum.Main", scope: null, file: !6, line: 16, type: !8, spFlags: DISPFlagDefinition, unit: !5)
-// CHECK:STDOUT: !16 = !DILocation(line: 17, column: 11, scope: !15)
-// CHECK:STDOUT: !17 = !DILocation(line: 17, column: 3, scope: !15)
-// CHECK:STDOUT: !18 = !DILocation(line: 16, column: 1, scope: !15)
+// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "ConvertEnumToI16", linkageName: "_CConvertEnumToI16.Main", scope: null, file: !6, line: 12, type: !13, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !16)
+// CHECK:STDOUT: !13 = !DISubroutineType(types: !14)
+// CHECK:STDOUT: !14 = !{null, !15}
+// CHECK:STDOUT: !15 = !DIBasicType(name: "int", size: 16, encoding: DW_ATE_signed)
+// CHECK:STDOUT: !16 = !{!17}
+// CHECK:STDOUT: !17 = !DILocalVariable(arg: 1, scope: !12, type: !15)
+// CHECK:STDOUT: !18 = !DILocation(line: 13, column: 3, scope: !12)
+// CHECK:STDOUT: !19 = !DILocation(line: 12, column: 1, scope: !12)
+// CHECK:STDOUT: !20 = distinct !DISubprogram(name: "ConvertI16ToEnum", linkageName: "_CConvertI16ToEnum.Main", scope: null, file: !6, line: 16, type: !13, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !21)
+// CHECK:STDOUT: !21 = !{!22}
+// CHECK:STDOUT: !22 = !DILocalVariable(arg: 1, scope: !20, type: !15)
+// CHECK:STDOUT: !23 = !DILocation(line: 17, column: 11, scope: !20)
+// CHECK:STDOUT: !24 = !DILocation(line: 17, column: 3, scope: !20)
+// CHECK:STDOUT: !25 = !DILocation(line: 16, column: 1, scope: !20)
 // CHECK:STDOUT: ; ModuleID = 'use_bitmask.carbon'
 // CHECK:STDOUT: source_filename = "use_bitmask.carbon"
 // CHECK:STDOUT: target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
@@ -275,6 +282,6 @@ fn Call() {
 // CHECK:STDOUT: !6 = !DIFile(filename: "use_bitmask.carbon", directory: "")
 // CHECK:STDOUT: !7 = distinct !DISubprogram(name: "Call", linkageName: "_CCall.Main", scope: null, file: !6, line: 10, type: !8, spFlags: DISPFlagDefinition, unit: !5)
 // CHECK:STDOUT: !8 = !DISubroutineType(types: !9)
-// CHECK:STDOUT: !9 = !{}
+// CHECK:STDOUT: !9 = !{null}
 // CHECK:STDOUT: !10 = !DILocation(line: 11, column: 3, scope: !7)
 // CHECK:STDOUT: !11 = !DILocation(line: 10, column: 1, scope: !7)