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/virtual_base.carbon

@@ -120,9 +120,9 @@ fn AccessD(d: Cpp.D) -> i32 {
 // CHECK:STDOUT: ; Function Attrs: nounwind
 // CHECK:STDOUT: define i32 @_CAccessD.Main(ptr %d) #0 !dbg !13 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %.loc16_11.1.d = getelementptr inbounds nuw [40 x i8], ptr %d, i32 0, i32 28, !dbg !14
-// CHECK:STDOUT:   %.loc16_11.2 = load i32, ptr %.loc16_11.1.d, align 4, !dbg !14
-// CHECK:STDOUT:   ret i32 %.loc16_11.2, !dbg !15
+// CHECK:STDOUT:   %.loc16_11.1.d = getelementptr inbounds nuw [40 x i8], ptr %d, i32 0, i32 28, !dbg !20
+// CHECK:STDOUT:   %.loc16_11.2 = load i32, ptr %.loc16_11.1.d, align 4, !dbg !20
+// CHECK:STDOUT:   ret i32 %.loc16_11.2, !dbg !21
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: ; Function Attrs: nocallback nofree nosync nounwind willreturn memory(argmem: readwrite)
@@ -188,10 +188,16 @@ fn AccessD(d: Cpp.D) -> i32 {
 // CHECK:STDOUT: !6 = !DIFile(filename: "use_diamond.carbon", directory: "")
 // CHECK:STDOUT: !7 = distinct !DISubprogram(name: "Make", linkageName: "_CMake.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: 11, column: 3, scope: !7)
 // CHECK:STDOUT: !11 = !DILocation(line: 11, column: 18, scope: !7)
 // CHECK:STDOUT: !12 = !DILocation(line: 6, column: 1, scope: !7)
-// CHECK:STDOUT: !13 = distinct !DISubprogram(name: "AccessD", linkageName: "_CAccessD.Main", scope: null, file: !6, line: 14, type: !8, spFlags: DISPFlagDefinition, unit: !5)
-// CHECK:STDOUT: !14 = !DILocation(line: 16, column: 10, scope: !13)
-// CHECK:STDOUT: !15 = !DILocation(line: 16, column: 3, scope: !13)
+// CHECK:STDOUT: !13 = distinct !DISubprogram(name: "AccessD", linkageName: "_CAccessD.Main", scope: null, file: !6, line: 14, type: !14, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !18)
+// CHECK:STDOUT: !14 = !DISubroutineType(types: !15)
+// CHECK:STDOUT: !15 = !{!16, !17}
+// CHECK:STDOUT: !16 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+// CHECK:STDOUT: !17 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 8)
+// CHECK:STDOUT: !18 = !{!19}
+// CHECK:STDOUT: !19 = !DILocalVariable(arg: 1, scope: !13, type: !17)
+// CHECK:STDOUT: !20 = !DILocation(line: 16, column: 10, scope: !13)
+// CHECK:STDOUT: !21 = !DILocation(line: 16, column: 3, scope: !13)