[clang] [CIR] Model cir.is_fp_class flags as a bit-enum (PR #205941)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I suspect we're going to dislike those aliases some day, as they make the mask itself kinda confusing.  But they already exist/is consistent with existing, but I can't imagine any of us will ever realize/remember on the first read that `fcNan` is actually 2 enums (despite being represented by 2 set bits, but 1 visual value).

This isn't a request to change this, more of an 'I told you so' in advance to the person who goes through git and says "why would we have done this nonsense" (probably future-erich).

https://github.com/llvm/llvm-project/pull/205941