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/primitives/string.carbon

@@ -69,8 +69,8 @@ fn Copy(s: str) -> str {
 // CHECK:STDOUT: ; Function Attrs: nounwind
 // CHECK:STDOUT: define void @_CCopy.Main(ptr sret({ ptr, i64 }) %return, ptr %s) #0 !dbg !15 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @"_COp.String.Core:Copy.Core"(ptr %return, ptr %s), !dbg !16
-// CHECK:STDOUT:   ret void, !dbg !17
+// CHECK:STDOUT:   call void @"_COp.String.Core:Copy.Core"(ptr %return, ptr %s), !dbg !21
+// CHECK:STDOUT:   ret void, !dbg !22
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: declare void @"_COp.String.Core:Copy.Core"(ptr sret({ ptr, i64 }), ptr)
@@ -86,7 +86,7 @@ fn Copy(s: str) -> str {
 // CHECK:STDOUT: !3 = !DIFile(filename: "string.carbon", directory: "")
 // CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
-// CHECK:STDOUT: !6 = !{}
+// CHECK:STDOUT: !6 = !{null}
 // CHECK:STDOUT: !7 = !DILocation(line: 19, column: 3, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 18, column: 1, scope: !4)
 // CHECK:STDOUT: !9 = distinct !DISubprogram(name: "H", linkageName: "_CH.Main", scope: null, file: !3, line: 22, type: !5, spFlags: DISPFlagDefinition, unit: !2)
@@ -95,6 +95,11 @@ fn Copy(s: str) -> str {
 // CHECK:STDOUT: !12 = distinct !DISubprogram(name: "I", linkageName: "_CI.Main", scope: null, file: !3, line: 28, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !13 = !DILocation(line: 31, column: 3, scope: !12)
 // CHECK:STDOUT: !14 = !DILocation(line: 28, column: 1, scope: !12)
-// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "Copy", linkageName: "_CCopy.Main", scope: null, file: !3, line: 34, type: !5, spFlags: DISPFlagDefinition, unit: !2)
-// CHECK:STDOUT: !16 = !DILocation(line: 35, column: 10, scope: !15)
-// CHECK:STDOUT: !17 = !DILocation(line: 35, column: 3, scope: !15)
+// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "Copy", linkageName: "_CCopy.Main", scope: null, file: !3, line: 34, type: !16, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !19)
+// CHECK:STDOUT: !16 = !DISubroutineType(types: !17)
+// CHECK:STDOUT: !17 = !{!18, !18}
+// CHECK:STDOUT: !18 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 8)
+// CHECK:STDOUT: !19 = !{!20}
+// CHECK:STDOUT: !20 = !DILocalVariable(arg: 1, scope: !15, type: !18)
+// CHECK:STDOUT: !21 = !DILocation(line: 35, column: 10, scope: !15)
+// CHECK:STDOUT: !22 = !DILocation(line: 35, column: 3, scope: !15)