[clang] [clang] implement CWG2064: ignore value dependence for decltype (PR #190495)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

FYI this patch is going to become significantly more complicated now that I realized this was attempted before, at https://github.com/llvm/llvm-project/commit/da986511fb9da1a46a0ca4dba2e49e2426036303, and the reason that was reverted needs to be addressed here.

https://github.com/llvm/llvm-project/pull/190495