[clang-tools-extra] [NFC] [clangd] [C++20] [Modules] Fix false duplicate module warning for equivalent paths (PR #199343)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

So this really should have had a test since this indicates a change in behavior and tests now only demonstrate the change but act as regression rests.

We also should have added a release note.

CC @ChuanqiXu9 

https://github.com/llvm/llvm-project/pull/199343