Standardize Enum.HasFlag

[Noggog]

Eventually I think we'll add a "fast" HasFlag option. On the back of a project/concept like this:
https://github.com/EngRajabi/Enum.Source.Generator
I dont know that that one specifically is good enough, but I might make one myself with our needs sometime.

and it'll be easier to find/replace HasFlag calls rather than the raw bit comparison alternative. If we have the raw alternative in the repo, new analyzers will copy the pattern and it'll be a bit of a mess