APFloat: x - NaN needs to flip the signbit of NaN when x is a number.

Because we don't have a separate negate( ) function, 0 - NaN does double-duty as the IEEE-754 negate( ) operation, which (unlike most FP ops) *does* attach semantic meaning to the signbit of NaN.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210428 91177308-0d34-0410-b5e6-96231b3b80d8