Fix APFloat from string conversion for Inf

The method IEEEFloat::convertFromStringSpecials() does not recognize
the "+Inf" and "-Inf" strings but these strings are printed for
the double Infinities by the IEEEFloat::toString().

This patch adds the "+Inf" and "-Inf" strings to the list of recognized
patterns in IEEEFloat::convertFromStringSpecials().

Re-landing after fix.

Reviewers: sberg, bogner, majnemer, timshen, rnk, skatkov, gottesmm, bkramer, scanon, anna
Reviewed By: anna
Subscribers: mkazantsev, FlameTop, llvm-commits, reames, apilipenko
Differential Revision: https://reviews.llvm.org/D38030

llvm-svn: 321054