Add support for importing a trivial global C++ function (#5033)

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.

toolchain/driver/compile_subcommand.cpp

@@ -412,6 +412,7 @@ class CompilationUnit {
   std::optional<std::function<auto()->const Parse::TreeAndSubtrees&>>
       tree_and_subtrees_getter_;
   std::optional<SemIR::File> sem_ir_;
+  std::unique_ptr<clang::ASTUnit> cpp_ast_;
   std::unique_ptr<llvm::LLVMContext> llvm_context_;
   std::unique_ptr<llvm::Module> module_;
 };
@@ -509,7 +510,8 @@ auto CompilationUnit::GetCheckUnit(SemIR::CheckIRId check_ir_id)
           .value_stores = &value_stores_,
           .timings = timings_ ? &*timings_ : nullptr,
           .tree_and_subtrees_getter = *tree_and_subtrees_getter_,
-          .sem_ir = &*sem_ir_};
+          .sem_ir = &*sem_ir_,
+          .cpp_ast = &cpp_ast_};
 }
 
 auto CompilationUnit::PostCheck() -> void {