[clang-tools-extra] [clangd] [C++20] [Modules] Add modules suffix for 'Header' Source Switch (PR #131591)

[Michael Spencer via cfe-commits]

Bigcheese wrote:

Is the expectation here that people will name implementation units "foo.cpp" and the corresponding interface unit "foo.cppm"? I guess that's reasonable.

https://github.com/llvm/llvm-project/pull/131591