[clang] [clang-tools-extra] [clangd] CodeCompletion surface param names if possible (PR #206716)

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 requested changes to this pull request.

Hmm, something doesn't seem quite right based on local testing.

In `IndexActionTest.DeclParamName`, if I comment out the first declaration (`void moon(int, int);`), the test now fails (suggesting that it's now using `void moon(int month, int night);`).

Based on the consistency claims made in the test comments, we'd still expect it to use `void moon(int, int day);`, right?

https://github.com/llvm/llvm-project/pull/206716