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

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

It might be useful to do what c++ reflection is doing here, ie only produce a name if all declarations introduce the same name (or no name)

https://eel.is/c++draft/meta.reflection.names#1.8
https://eel.is/c++draft/meta.reflection.names#3.3

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