[libc] Add a class called NormalFloat which represents normalized floats.

This class helps in dealing with normal and subnormal numbers uniformly.
Moreover, since this class has been designed to handle all floating
formats across platforms, it helps implement floating point functions in
a uniform manner.

The implementations of frexp and logb have been switched to use this new
class as it allows us to use just one implementation across all
different floating point formats.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D86241