[llvm] [KnownFPClass] Refactor NaN sign encoding for KnownFPClass (PR #220606)

[Tim Gymnich via llvm-commits]

tgymnich wrote:

> Should we also change FPClassTest to use LLVM_MARK_AS_BITMASK_ENUM?

I would not touch `FPClassTest` if we don't need to.

https://github.com/llvm/llvm-project/pull/220606