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

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 commented:

I prefer to change the term "explicit module" in the code to something like `CXX20 module` or `named module` or whatever. Since when I read the term `explicit module` I assumed it is a style to build clang header modules. 

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