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

[via cfe-commits]

h-vetinari wrote:

The branch-point for LLVM 19 is about 2 months away ([July 23rd](https://discourse.llvm.org/t/18-1-6-released/79068)), in case this context helps to plan/prioritize reviews here.

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