[Carbon/C++ interop] Add support for C++ overloaded functions (#5891)

As proposed in [Carbon: C++ interop for overloaded functions and
function
templates](https://docs.google.com/document/d/1KUxumZtNe3mY3TsjW2s_ZADOlAaFlrtsLKHVILtqIaM/edit?tab=t.0),
Clang is used to perform the overload resolution using C++ rules, when
an overloaded C++ set is called from Carbon. Once a function is
selected, it's converted into a Carbon function and called using the
Carbon rules including argument conversions.

A single non-templated function is treated the same way as an overload
set and the same rules apply for its call.
Template functions are not supported yet.

Demo:

a) Non-templated function calls:

```c++
// --- overloads.h

auto foo(int a, short b) -> void;
auto foo(double a) -> void;
auto foo(int a) -> void;
```

```c++
// overloads.cpp

#include "overloads.h"
#include <cstdio>

auto foo(int a, short b) -> void {
  printf("hello from foo_int_short(%d, %d) \n", a, b);
}
auto foo(double a) -> void { printf("hello from foo_double(%f) \n", a); }
auto foo(int a) -> void { printf("hello from foo_int(%d) \n", a); }
```
```c++
library "Main";

import Cpp library "overloads.h";

fn Run() -> i32 {
  Cpp.foo(1.1 as f64);
  return 0;
}
```
```
$ clang -c overloads.cpp 
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link overloads.o main.o --output=demo
$ ./demo
hello from foo_double(1.100000) 
```

b) Constructors:
```c++
// --- constructor_overloads.h
class C {
 public:
  C();
  C(int a, int b);
};
```

```c++
// constructor_overloads.cpp
#include "constructor_overloads.h"
#include <cstdio>

C::C() { printf("hello from C() \n"); }
C::C(int a, int b) { printf("hello from C(%d, %d) \n", a, b); }
```
```c++
library "Main";

import Cpp library "constructor_overloads.h";

fn Run() -> i32 {
  let c1: Cpp.C = Cpp.C.C();
  let c2: Cpp.C = Cpp.C.C(1, 2);
  return 0;
}
```

```
$ clang -c constructor_overloads.cpp 
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link constructor_overloads.o main.o \--output=demo
$ ./demo
hello from C() 
hello from C(1, 2) 
```


Follow-ups:

- `Cpp.foo({})` - proper handling of struct literals as call args.
- Fix access for overloaded sets.
- Fix tests:
- Method calls: `error: missing object argument in method call
[MissingObjectInMethodCall]` in tests.
    - Fix `toolchain/check/testdata/interop/cpp/import.carbon` test.
    - Fix `enums` support.
    - Fix `str` -> `std::string_view` mapping.


Part of #5915

toolchain/check/testdata/interop/cpp/class/struct.carbon

@@ -179,10 +179,10 @@ import Cpp library "dynamic.h";
 fn MyF(bar: Cpp.Bar*) {
   // CHECK:STDERR: fail_todo_call_dynamic.carbon:[[@LINE+7]]:3: error: semantics TODO: `Unsupported: Virtual function` [SemanticsTodo]
   // CHECK:STDERR:   bar->f();
-  // CHECK:STDERR:   ^~~~~~
-  // CHECK:STDERR: fail_todo_call_dynamic.carbon:[[@LINE+4]]:3: note: in `Cpp` name lookup for `f` [InCppNameLookup]
+  // CHECK:STDERR:   ^~~~~~~~
+  // CHECK:STDERR: fail_todo_call_dynamic.carbon:[[@LINE+4]]:3: note: in call to Cpp function here [InCallToCppFunction]
   // CHECK:STDERR:   bar->f();
-  // CHECK:STDERR:   ^~~~~~
+  // CHECK:STDERR:   ^~~~~~~~
   // CHECK:STDERR:
   bar->f();
 }
@@ -325,6 +325,8 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT: constants {
 // CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
 // CHECK:STDOUT:   %Bar: type = class_type @Bar [concrete]
+// CHECK:STDOUT:   %.c07: type = cpp_overload_set_type @Bar.foo [concrete]
+// CHECK:STDOUT:   %empty_struct: %.c07 = struct_value () [concrete]
 // CHECK:STDOUT:   %Bar.foo.type: type = fn_type @Bar.foo [concrete]
 // CHECK:STDOUT:   %Bar.foo: %Bar.foo.type = struct_value () [concrete]
 // CHECK:STDOUT: }
@@ -335,6 +337,7 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT:     import Cpp//...
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Bar.decl: type = class_decl @Bar [concrete = constants.%Bar] {} {}
+// CHECK:STDOUT:   %.617: %.c07 = cpp_overload_set_value @Bar.foo [concrete = constants.%empty_struct]
 // CHECK:STDOUT:   %Bar.foo.decl: %Bar.foo.type = fn_decl @Bar.foo [concrete = constants.%Bar.foo] {} {}
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -342,8 +345,8 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %Cpp.ref: <namespace> = name_ref Cpp, imports.%Cpp [concrete = imports.%Cpp]
 // CHECK:STDOUT:   %Bar.ref: type = name_ref Bar, imports.%Bar.decl [concrete = constants.%Bar]
-// CHECK:STDOUT:   %foo.ref: %Bar.foo.type = name_ref foo, imports.%Bar.foo.decl [concrete = constants.%Bar.foo]
-// CHECK:STDOUT:   %Bar.foo.call: init %empty_tuple.type = call %foo.ref()
+// CHECK:STDOUT:   %foo.ref: %.c07 = name_ref foo, imports.%.617 [concrete = constants.%empty_struct]
+// CHECK:STDOUT:   %Bar.foo.call: init %empty_tuple.type = call imports.%Bar.foo.decl()
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -478,6 +481,8 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT:   %pattern_type: type = pattern_type %ptr.f68 [concrete]
 // CHECK:STDOUT:   %MyF.type: type = fn_type @MyF [concrete]
 // CHECK:STDOUT:   %MyF: %MyF.type = struct_value () [concrete]
+// CHECK:STDOUT:   %.2e2: type = cpp_overload_set_type @<null name> [concrete]
+// CHECK:STDOUT:   %empty_struct: %.2e2 = struct_value () [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: imports {
@@ -486,6 +491,7 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT:     import Cpp//...
 // CHECK:STDOUT:   }
 // CHECK:STDOUT:   %Bar.decl: type = class_decl @Bar [concrete = constants.%Bar] {} {}
+// CHECK:STDOUT:   %.6e0: %.2e2 = cpp_overload_set_value @<null name> [concrete = constants.%empty_struct]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: file {
@@ -507,7 +513,7 @@ fn MyF(bar: Cpp.Bar*);
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %bar.ref: %ptr.f68 = name_ref bar, %bar
 // CHECK:STDOUT:   %.loc15: ref %Bar = deref %bar.ref
-// CHECK:STDOUT:   %f.ref: <error> = name_ref f, <error> [concrete = <error>]
+// CHECK:STDOUT:   %f.ref: %.2e2 = name_ref f, imports.%.6e0 [concrete = constants.%empty_struct]
 // CHECK:STDOUT:   return
 // CHECK:STDOUT: }
 // CHECK:STDOUT: