[clang] [clang-tools-extra] [libcxx] [lldb] [llvm] Add clang basic module directory (PR #93388)

[Chuanqi Xu via llvm-commits]

ChuanqiXu9 wrote:

BTW, I tried to split `Module` class into `ModuleBase`, `ClangModule` and `Cpp20Modules` (and HeaderUnits) classes to improve the readability. But it showed too hard and too many things get changes then I stopped.

https://github.com/llvm/llvm-project/pull/93388