Boaz Brickner 156ab889f8 Support mangling imported C++ functions using Clang's MangleContext (#5062) ...

Keep a pointer to the Clang declaration in Carbon's function declaration
and use it in Carbon mangling by calling Clang mangling.
Create Clang's `MangleContext` once on demand.

Part of #4666.

C++ Interop Demo:

```c++
// hello_world.h

void hello_world();
```

```c++
// hello_world.cpp

#include <cstdio>

void hello_world() { printf("Hello World!\n"); }
```

```
// main.carbon

library "Main";

import Cpp library "hello_world.h";

fn Run() -> i32 {
  Cpp.hello_world();
  return 0;
}
```

```shell
$ clang -c hello_world.cpp
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link hello_world.o main.o --output=demo
$ ./demo
Hello World!
```

2025-03-06 09:51:01 +00:00

..

function_decl.carbon

Support mangling imported C++ functions using Clang's MangleContext (#5062)

2025-03-06 09:51:01 +00:00