[Driver] Enable to use C++20 standalne by -fcxx-modules

This patch allows user to use C++20 module by -fcxx-modules. Previously,
we could only use it under -std=c++20. Given that user could use C++20
coroutine standalonel by -fcoroutines-ts. It makes sense to offer an
option to use C++20 modules without enabling C++20.

Reviewed By: iains, MaskRay

Differential Revision: https://reviews.llvm.org/D120540