[clang-tools-extra] [clangd] [C++20] [Modules] Introduce initial support for C++20 Modules (PR #66462)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

@sam-mccall thanks  for your high qualified comments! I think I've addressed most of them. I am not so familiar with the Github review system. Tell me if you're more happy with other practices.

https://github.com/llvm/llvm-project/pull/66462