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/function_decl.carbon

@@ -159,7 +159,7 @@ fn MyF() {
 // CHECK:STDOUT: !6 = !DIFile(filename: "import_function_decl.carbon", directory: "")
 // CHECK:STDOUT: !7 = distinct !DISubprogram(name: "MyF", linkageName: "_CMyF.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: 3, scope: !7)
 // CHECK:STDOUT: !11 = !DILocation(line: 6, column: 1, scope: !7)
 // CHECK:STDOUT: ; ModuleID = 'import_inline_function_decl.carbon'
@@ -197,7 +197,7 @@ fn MyF() {
 // CHECK:STDOUT: !6 = !DIFile(filename: "import_inline_function_decl.carbon", directory: "")
 // CHECK:STDOUT: !7 = distinct !DISubprogram(name: "MyF", linkageName: "_CMyF.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: 3, scope: !7)
 // CHECK:STDOUT: !11 = !DILocation(line: 6, column: 1, scope: !7)
 // CHECK:STDOUT: ; ModuleID = 'import_inline_used_function_decl.carbon'
@@ -240,7 +240,7 @@ fn MyF() {
 // CHECK:STDOUT: !6 = !DIFile(filename: "import_inline_used_function_decl.carbon", directory: "")
 // CHECK:STDOUT: !7 = distinct !DISubprogram(name: "MyF", linkageName: "_CMyF.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: 3, scope: !7)
 // CHECK:STDOUT: !11 = !DILocation(line: 6, column: 1, scope: !7)
 // CHECK:STDOUT: ; ModuleID = 'import_multiple_inline_function_decls.carbon'
@@ -296,7 +296,7 @@ fn MyF() {
 // CHECK:STDOUT: !6 = !DIFile(filename: "import_multiple_inline_function_decls.carbon", directory: "")
 // CHECK:STDOUT: !7 = distinct !DISubprogram(name: "MyF", linkageName: "_CMyF.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: 3, scope: !7)
 // CHECK:STDOUT: !11 = !DILocation(line: 8, column: 3, scope: !7)
 // CHECK:STDOUT: !12 = !DILocation(line: 9, column: 3, scope: !7)
@@ -346,7 +346,7 @@ fn MyF() {
 // CHECK:STDOUT: !6 = !DIFile(filename: "import_inline_recursive_function_decl.carbon", directory: "")
 // CHECK:STDOUT: !7 = distinct !DISubprogram(name: "MyF", linkageName: "_CMyF.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: 8, column: 3, scope: !7)
 // CHECK:STDOUT: !11 = !DILocation(line: 6, column: 1, scope: !7)
 // CHECK:STDOUT: ; ModuleID = 'call_with_default_args.carbon'
@@ -428,7 +428,7 @@ fn MyF() {
 // CHECK:STDOUT: !6 = !DIFile(filename: "call_with_default_args.carbon", directory: "")
 // CHECK:STDOUT: !7 = distinct !DISubprogram(name: "MyF", linkageName: "_CMyF.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: 7, column: 3, scope: !7)
 // CHECK:STDOUT: !12 = !DILocation(line: 8, column: 3, scope: !7)