[clangd] Make forwarding parameter detection logic resilient

This could crash when our heuristic picks the wrong function. Make sure
there is enough parameters in the candidate to prevent those crashes.

Also special case copy/move constructors to make the heuristic work in
presence of those.

Fixes https://github.com/llvm/llvm-project/issues/56620

Differential Revision: https://reviews.llvm.org/D130260