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

[kadir çetinkaya via cfe-commits]

https://github.com/kadircet requested changes to this pull request.

thanks a lot and sorry for taking so long.

i think this looks great at a high level, mostly asked for some changes in the implementation. the only layering-wise change is who should own `ModulesBuilder` (you can find details in the specific comment thread).

apart from that one, i think unittests can benefit from some "virtualizaiton", i believe things become a lot more simpler, if we ignore all the other module related flags that needs to be provided.

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