[clang] [clang][modules-driver] Add scanner to detect C++20 module presence (PR #145220)

[Naveen Seth Hanig via cfe-commits]

naveen-seth wrote:

Done. For the regression test, I've updated it to use `--allow-empty` instead of forcing output using `clang -v` (+formatting & removed duplicate flags). Thank you for the reviews.

https://github.com/llvm/llvm-project/pull/145220