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

[via cfe-commits]

Sirraide wrote:

> Since that needs to happen at the end of the TU I need a work list to avoid a recursive AST visitor. FlagBitsCache can be re-used for that, we just need to make sure it's populated

Ok, I think that makes sense. In that case we need to make sure that 1. we actually call that for every enum (including instantiations) and 2. that we don’t try to query whether something is a flag enum before we had a chance to add it to the cache (currently, this might be possible if the underlying type is dependent, as we don’t get to the line that adds the enum to the cache in that case)

https://github.com/llvm/llvm-project/pull/210632