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

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> that being said AST and Index will still be consistent so [...] I do not think it changes anything for this PR

Unfortunately, that's not what I'm seeing in local testing:

<img width="781" height="471" alt="param-names-1" src="https://github.com/user-attachments/assets/8d2bc7f2-4fd5-49d9-8d3f-d0f86cae4782" />

versus:

<img width="1071" height="475" alt="param-names-2" src="https://github.com/user-attachments/assets/f4cae9e4-7306-448b-a8b6-87709a37c894" />

We even manage to be inconsistent between completion and signature help on the same code:

<img width="1073" height="467" alt="param-names-3" src="https://github.com/user-attachments/assets/936214d3-ba9f-479d-9e77-010fad397aae" />


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