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().

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

llvm-svn: 316156