[APFloat] Make all arithmetic operations with NaN produce positive NaNs.

IEEE-754R 1.4 Exclusions states that IEEE-754R does not specify the
interpretation of the sign of NaNs. In order to remove an irrelevant
variable that most floating point implementations do not use,
standardize add, sub, mul, div, mod so that operating anything with
NaN always yields a positive NaN.

In a later commit I am going to update the APIs for creating NaNs so
that one can not even create a negative NaN.

llvm-svn: 187314

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