[clang-tools-extra] [clangd] [C++ Modules] Fix handling of relative paths in prebuilt mod… (PR #187654)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

I'd like to maintain clangd's modules support and I am the original authors for it too. And the patch won't impact any clangd users who don't use the experimental C++20 modules support. So I'd like to land the patch one week later if no more comments came in.

https://github.com/llvm/llvm-project/pull/187654