[APFloat] Converted all references to APFloat::isNormal => APFloat::isFiniteNonZero.

Turns out all the references were in llvm and not in clang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184356 91177308-0d34-0410-b5e6-96231b3b80d8