[clang] Revert "[clang][NFC] Refactor flag enum caching (#210632)" (PR #223244)

[Matthias Wippich via cfe-commits]

Tsche wrote:

> I'm afraid I don't know this code at all.

No worries, if you need me to take over just let me know - I might be able to cram it into next week's schedule.

> IMO not breaking existing code takes precedence. Maybe I'm biased because it's my code that's getting broken

Sure but if there's ways to avoid undoing work, we should explore them - especially given that this refactor was in preparation for a larger, more interesting patch. Reverting isn't the only way to deal with unintentional breaks :P

https://github.com/llvm/llvm-project/pull/223244