[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 approved this pull request.

thanks a lot, LGTM!

i think the only big item remaining is injecting resource-dir correctly when running clang-scan-deps.

the most important follow up (for functionality, rather than optimizations and usability) is indexing the PCMs as we build them (similar to preamble-indexing we run today). For any future travelers, please do start a discussion in advance for that one.

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