[APFloat] Enlarge ExponentType to 32bit integer

Enlarge the size of ExponentType from 16bit integer to 32bit. This is
required to prevent exponent overflow/underflow.

Note that IEEEFloat size and alignment don't change in 64bit or 32bit
compilation targets (and in turn, neither does APFloat).

Fixes PR34851.

Differential Revision: https://reviews.llvm.org/D69771