[APFloat] Extend conversion from special strings

Add support for converting Signaling NaN, and a NaN Payload from string.

The NaNs (the string "nan" or "NaN") may be prefixed with 's' or 'S' for defining a Signaling NaN.

A payload for a NaN can be specified as a suffix.
It may be a octal/decimal/hexadecimal number in parentheses or without.

Differential Revision: https://reviews.llvm.org/D69773