[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 I created https://github.com/llvm/llvm-project/pull/73483 as the following patches to reuse built module files. I think that patch should be necessary since the current patch may waste too many time and space since it won't reuse the module files across source files. I am wondering if we can land the current patch if it doesn't have big issues. Then we can turn into the next iterations.

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