[APFloat] Rename llvm::exponent_t => llvm::APFloat::ExponentType.

exponent_t is only used internally in APFloat and no exponent_t values are
exposed via the APFloat API. In light of such conditions it does not make any
sense to gum up the llvm namespace with said type. Plus it makes it clearer that
exponent_t is associated with APFloat.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184686 91177308-0d34-0410-b5e6-96231b3b80d8
[APFloat] Removed out of date comment from isNormal().

I already finished the isIEEENormal => isNormal transition. So isNormal is now
IEEE-754R compliant.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184687 91177308-0d34-0410-b5e6-96231b3b80d8