Resubmit "Add support for advanced number formatting."

This resubmits r284436 and r284437, which were reverted in
r284462 as they were breaking the AArch64 buildbot.

The breakage on AArch64 turned out to be a miscompile which is
still not fixed, but is actively tracked at llvm.org/pr30748.

This resubmission re-writes the code in a way so as to make the
miscompile not happen.

llvm-svn: 285483