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

[Matthias Wippich via cfe-commits]

Tsche wrote:

Just to be clear: I'm not saying "definitely don't revert this". 

I'm saying that I believe there's a much less intrusive way to address this break and I do not feel comfortable approving a revert before at least exploring that possibility.

Others may feel differently and that's fine, a revert isn't the end of the world and I'm not the only one who may approve this either. Just letting you know why I haven't done that yet.

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