Substantially optimize APFloat::toString() by doing a single large divide to

cut the significand down to the desired precision *before* entering the
core divmod loop.  Makes the overall algorithm logarithmic in the exponent.

There's still a lot of room for improvement here, but this gets the
performance back down to acceptable-for-diagnostics levels, even for
long doubles.
negligible, even on long doubles.

llvm-svn: 92130