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/if_expr/basic.carbon

@@ -23,31 +23,31 @@ fn Select(b: bool) -> i32 {
 // CHECK:STDOUT: ; Function Attrs: nounwind
 // CHECK:STDOUT: define i32 @_CF.Main() #0 !dbg !4 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   ret i32 1, !dbg !7
+// CHECK:STDOUT:   ret i32 1, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nounwind
-// CHECK:STDOUT: define i32 @_CG.Main() #0 !dbg !8 {
+// CHECK:STDOUT: define i32 @_CG.Main() #0 !dbg !9 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   ret i32 2, !dbg !9
+// CHECK:STDOUT:   ret i32 2, !dbg !10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nounwind
-// CHECK:STDOUT: define i32 @_CSelect.Main(i1 %b) #0 !dbg !10 {
+// CHECK:STDOUT: define i32 @_CSelect.Main(i1 %b) #0 !dbg !11 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   br i1 %b, label %if.expr.then, label %if.expr.else, !dbg !11
+// CHECK:STDOUT:   br i1 %b, label %if.expr.then, label %if.expr.else, !dbg !17
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.expr.then:                                     ; preds = %entry
-// CHECK:STDOUT:   %F.call = call i32 @_CF.Main(), !dbg !12
-// CHECK:STDOUT:   br label %if.expr.result, !dbg !11
+// CHECK:STDOUT:   %F.call = call i32 @_CF.Main(), !dbg !18
+// CHECK:STDOUT:   br label %if.expr.result, !dbg !17
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.expr.else:                                     ; preds = %entry
-// CHECK:STDOUT:   %G.call = call i32 @_CG.Main(), !dbg !13
-// CHECK:STDOUT:   br label %if.expr.result, !dbg !11
+// CHECK:STDOUT:   %G.call = call i32 @_CG.Main(), !dbg !19
+// CHECK:STDOUT:   br label %if.expr.result, !dbg !17
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.expr.result:                                   ; preds = %if.expr.else, %if.expr.then
 // CHECK:STDOUT:   %0 = phi i32 [ %F.call, %if.expr.then ], [ %G.call, %if.expr.else ]
-// CHECK:STDOUT:   ret i32 %0, !dbg !14
+// CHECK:STDOUT:   ret i32 %0, !dbg !20
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: attributes #0 = { nounwind }
@@ -61,12 +61,18 @@ fn Select(b: bool) -> i32 {
 // CHECK:STDOUT: !3 = !DIFile(filename: "basic.carbon", directory: "")
 // CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
-// CHECK:STDOUT: !6 = !{}
-// CHECK:STDOUT: !7 = !DILocation(line: 13, column: 17, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
-// CHECK:STDOUT: !9 = !DILocation(line: 14, column: 17, scope: !8)
-// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "Select", linkageName: "_CSelect.Main", scope: null, file: !3, line: 16, type: !5, spFlags: DISPFlagDefinition, unit: !2)
-// CHECK:STDOUT: !11 = !DILocation(line: 17, column: 10, scope: !10)
-// CHECK:STDOUT: !12 = !DILocation(line: 17, column: 20, scope: !10)
-// CHECK:STDOUT: !13 = !DILocation(line: 17, column: 29, scope: !10)
-// CHECK:STDOUT: !14 = !DILocation(line: 17, column: 3, scope: !10)
+// CHECK:STDOUT: !6 = !{!7}
+// CHECK:STDOUT: !7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+// CHECK:STDOUT: !8 = !DILocation(line: 13, column: 17, scope: !4)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !10 = !DILocation(line: 14, column: 17, scope: !9)
+// CHECK:STDOUT: !11 = distinct !DISubprogram(name: "Select", linkageName: "_CSelect.Main", scope: null, file: !3, line: 16, type: !12, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !15)
+// CHECK:STDOUT: !12 = !DISubroutineType(types: !13)
+// CHECK:STDOUT: !13 = !{!7, !14}
+// CHECK:STDOUT: !14 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 8)
+// CHECK:STDOUT: !15 = !{!16}
+// CHECK:STDOUT: !16 = !DILocalVariable(arg: 1, scope: !11, type: !14)
+// CHECK:STDOUT: !17 = !DILocation(line: 17, column: 10, scope: !11)
+// CHECK:STDOUT: !18 = !DILocation(line: 17, column: 20, scope: !11)
+// CHECK:STDOUT: !19 = !DILocation(line: 17, column: 29, scope: !11)
+// CHECK:STDOUT: !20 = !DILocation(line: 17, column: 3, scope: !11)