[clang] [AArch64] Refactor implementation of FP8 types (NFC) (PR #123604)

[Timm Baeder via cfe-commits]

tbaederr wrote:

Is `__mfp8` a floating type? `isFloatingType()` on it returns `false`, was that the case before as well?

https://github.com/llvm/llvm-project/pull/123604