[clang] Reland: [clang] unified CWG2398 and P0522 changes; finishes implementation of P3310 (PR #124137)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Can you explain the revert-issue, and what you did to fix it?  The github diff is making REALLY difficult to understand what you did..

https://github.com/llvm/llvm-project/pull/124137