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

[Sam McCall via cfe-commits]

https://github.com/sam-mccall commented:

Thanks, this looks really close to me!
Tried to focus on how the components fit together, the details of module building and reuse can easily be tweaked later if needed.

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