[clang] [Clang] Reapply CWG2369 "Ordering between constraints and substitution" (PR #122423)

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> Can we have this behavior guarded by a compiler option?

That depends on the rationale:
1. Allow opt in to the resolution of CWG2369 because it is wanted (despite potentially-temporary fallout); or
2. Require opt in to the resolution of CWG2369 because "it causes fallout".

For (2), we can simply delay enabling the resolution until the fallout is mitigated or accepted.

https://github.com/llvm/llvm-project/pull/122423