This requires:
* Making `FunctionDecl` mutable since generating code
(`HandleTopLevelDecl()`) requires a mutable declaration and since we
manually add `used` attribute to force code generation.
* Passing the file system to `Lower` since it's needed by Clang code
generation.
* Creating an internal Clang LLVM module and link it against the Carbon
LLVM module.
Demo:
```c++
// hello_world.h
extern int puts;
inline void hello_world() {
((int (*)(const char*))&puts)("hello world");
}
```
```carbon
// main.carbon
library "Main";
import Cpp library "hello_world.h";
fn Run() -> i32 {
Cpp.hello_world();
return 0;
}
```
```shell
$ bazel-bin/toolchain/carbon compile main.carbon
$ bazel-bin/toolchain/carbon link main.o --output=demo
$ ./demo
hello world
```
Based on https://github.com/carbon-language/carbon-lang/pull/5406.
Part of #5405.
ASTUnit is owned by `CompileSubcommand`, passed through `Unit` to be
populated in `ImportCppFiles()` and used via `SemIR::File`.
When generating the AST, pass `-x c++` args to compile C++ (temporary
until we pass args properly).
`Cpp` namespace is marked as a special namespace and has dedicated logic
in `LookupNameInExactScope()`.
The logic for importing declarations from C++ to Carbon is in
`import_cpp.cpp`, but we're likely to want to refactor this
signfiicantly over time as it grows (perhaps a dedicated directory?).
Part of #4666.
Boaz Brickner