This adds support for having different C++ `NameScope`s (and not just
the main `Cpp` scope), and we keep a pointer to `clang::DeclContext`
these scopes so we can look up C++ names in the right part of the AST.
C++ Interop Demo with a namespace:
```c++
// hello_world.h
namespace some_namespace {
void hello_world();
} // namespace some_namespace
```
```c++
// hello_world.cpp
#include <cstdio>
namespace some_namespace {
void hello_world() { printf("Hello World!\n"); }
} // namespace some_namespace
```
```carbon
// main.carbon
library "Main";
import Cpp library "hello_world.h";
fn Run() -> i32 {
Cpp.some_namespace.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!
```
Closes#5102
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
Richard Smith