[libc][NFC] Decouple FP properties from C++ types (#73517)

We simplify the floating point properties by splitting concerns:
 - We define all distinct floating point formats in the `FPType` enum.
 - We map them to properties via the `FPProperties` trait.
- We map from C++ type to `FPType` in the `getFPType` function so logic
is easier to understand and extend.