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

[Guillaume Chatelet via llvm-commits]

gchatelet wrote:

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

Thx! For now I _fixed it_ with a `static_cast` and a TODO. I'll address it in a follow up patch.

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