This patch adds three options for printf decimal long doubles, and these
can also apply to doubles.
1. Use a giant table which is fast and accurate, but takes up ~5MB).
2. Use dyadic floats for approximations, which only gives ~50 digits of
accuracy but is very fast.
3. Use large integers for approximations, which is accurate but very
slow.
Reviewed By: sivachandra, lntue
Differential Revision: https://reviews.llvm.org/D150399
