[clang] [Clang] Improve concept performance 1/N (PR #188421)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

> Results from my concept heavy codebase (hmmm looks like it wasn't a large of a regression as last time I checked, maybe the other patches you landed helped as well?). Regardless, it def helped! (fsyntax-only builds, vanilla non pgo/lto/bolt binaries)

Yes, there were a few patches before that. However this one is probably the most impactful.

https://github.com/llvm/llvm-project/pull/188421