[libc] [llvm] [reland][libc][NFC] Refactor FPBits and remove LongDoubleBits specialization (PR #78447)

[Clement Courbet via llvm-commits]

legrosbuffle wrote:

As discussed, `create_value(bool sign, StorageType biased_exp, StorageType mantissa)`  should be taking a `BiasedExponent`.

https://github.com/llvm/llvm-project/pull/78447