Removed potential buffer overrun (spotted by Neil Booth) when NumericLiteralParser

converts a parsed literal into an APFloat. We are still performing a copy of the
string, which hopefully will be removed eventually for performance reasons. This
version now is at least safe.

Changed rounding in APFloat construction in NumericLiteralParser from rmTowardsZero
to rmNearestTiesToEven.

llvm-svn: 44422