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

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timon-ul wrote:

> But the inconsistency between code completion and signature help is still there.

Oops I was so focused on fixing the discrepency between AST and Index that I forgot to look at that.

> Ok, it seems to be "start with the first declaration, then walk backwards".

Yes, as I mentioned from my test above, `redecls()` walks backwards (1->5->4->3->2). Nothing we can really do against this wtithout introducing a lot more work. We could start with the last declaration if you prefer but even that seems to be more work and I think the current behaviour is fine (because it is so simple) until someone comes up with a reason why it isn't.

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