[C++20] [Modules] Prevent to accept clang modules

Close https://github.com/llvm/llvm-project/issues/64755

This wouldn't affect the form @import as the test shows. The two
affected test case `diag-flags.cpp` and `diag-pragma.cpp` are old test
cases in 2017 and 2018, when we're not so clear about the direction of
modules. And the things that these 2 tests tested can be covered by
clang modules naturally. So I change the them into clang modules to
not block this patch.