[libc-commits] [libc] [libc][NFC] Rename `MANTISSA_WIDTH` in `FRACTION_BITS` (PR #75489)

[Guillaume Chatelet via libc-commits]

gchatelet wrote:

@lntue 

I've made the changes for all of the properties at once.

I've also fixed various things like "unnecessary casts" as individual commits above. You may want to check them but they should all be NFC.

I still kind of like the `_BITS` suffix better but I'm not opposed to `_LEN` so it's up to you.
Let me know what you think!

https://github.com/llvm/llvm-project/pull/75489