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

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

> I highly recommend talking to @mathstuf of the CMake project about a way for the build system can emit a next generation set of build graph metadata (think modules appropriate compile commands, including dependencies).
> 
> I expect it will be more efficient for the build system, compiler, and LSP to all work together compared to implementing large subsets of a build system in the clangd LSP.

Yeah, yeah, of course. This is listed as the second step of `What we need to do next` section. And for the patch itself, it is just an initial move in the clangd for C++20 modules. Since it is complex so that we decide to split the works into several patches to make sure every one could be in the same page. I've already left the space to extend how we find the map from module name to source files later. But now I prefer to not change the design even if it looks unefficient clearly.

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