[clang] [clang-tools-extra] [lldb] [clang] implement CWG1980 and CWG2064: instantiation-dependency improvements (PR #190495)

[Erich Keane via cfe-commits]

erichkeane wrote:

This is sizable enough that I haven't had the time to dedicate to it yet, so I'd still like a chance to make time to review this.  I'll do a 'quick' pass right now, but need to spend some time on it, due to its size.

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