[libc] [llvm] [libc][NFC] Refactor FPBits and remove LongDoubleBits specialization (PR #78192)

[Guillaume Chatelet via llvm-commits]

gchatelet wrote:

Thank you so much for the detailed investigation @nickdesaulniers !
I can log on the build bot and start fixing the patch.

https://github.com/llvm/llvm-project/pull/78192