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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92130 91177308-0d34-0410-b5e6-96231b3b80d8