Factor out a Check::CppContext holding C++-specific check state (#6482)

* Move `Sema` access from `CppFile` into `CppContext`.
* Move the mangle context from `SemIR::File` into `CppContext`.
* Move source location mapping state from `Context` into `CppContext`.

Also factor out the `GenerateAst` function that builds the `CppContext`
and `CppFile` into its own file.

toolchain/check/cpp/thunk.cpp

@@ -349,7 +349,7 @@ static auto CreateThunkFunctionDecl(
   thunk_function_decl->addAttr(clang::AsmLabelAttr::CreateImplicit(
       ast_context,
       GenerateThunkMangledName(
-          *context.sem_ir().clang_mangle_context(), *callee_info.decl,
+          context.cpp_context()->clang_mangle_context(), *callee_info.decl,
           callee_info.num_params - callee_info.has_explicit_object_parameter()),
       clang_loc));