[clang-tools-extra] [clangd] Add test for GH75115 (PR #75116)

[Nico Weber via cfe-commits]

nico wrote:

Seems like unnecessary complexity to me fwiw. So far, we've put tests for things that work in the test suite and put a record of things that assert in the bug tracker :)

https://github.com/llvm/llvm-project/pull/75116