[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/sem_ir/function.cpp

@@ -9,6 +9,7 @@
 #include "toolchain/sem_ir/file.h"
 #include "toolchain/sem_ir/generic.h"
 #include "toolchain/sem_ir/ids.h"
+#include "toolchain/sem_ir/typed_insts.h"
 
 namespace Carbon::SemIR {
 
@@ -19,7 +20,8 @@ auto GetCalleeFunction(const File& sem_ir, InstId callee_id,
                            .resolved_specific_id = SpecificId::None,
                            .self_type_id = InstId::None,
                            .self_id = InstId::None,
-                           .is_error = false};
+                           .is_error = false,
+                           .is_cpp_overload_set = false};
   if (auto bound_method = sem_ir.insts().TryGetAs<BoundMethod>(callee_id)) {
     result.self_id = bound_method->object_id;
     callee_id = bound_method->function_decl_id;
@@ -46,6 +48,12 @@ auto GetCalleeFunction(const File& sem_ir, InstId callee_id,
   auto fn_type_inst =
       sem_ir.types().GetAsInst(sem_ir.insts().Get(val_id).type_id());
 
+  if (fn_type_inst.TryAs<CppOverloadSetType>()) {
+    // TODO: Consider evaluating this at runtime instead of having a field.
+    result.is_cpp_overload_set = true;
+    return result;
+  }
+
   if (auto impl_fn_type = fn_type_inst.TryAs<FunctionTypeWithSelfType>()) {
     // Combine the associated function's `Self` with the interface function
     // data.