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

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> 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).

I had similar thoughts regarding how then end up getting printed. They are useful for implementing the `__builtin_fpclassify`, which groups the tests at this granularity.

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