[Support] Add support for "advanced" number formatting.

raw_ostream has not afforded a lot of flexibility in terms of
how to format numbers when outputting.  Wrap this all up into
a set of low level helper functions that can be used to output
numbers with arbitrary precision, alignment, format, etc and
then update raw_ostream to use these functions.

This will be useful for upcoming improvements to llvm's string
formatting libraries, but are still useful independently.

Differential Revision: https://reviews.llvm.org/D25497

llvm-svn: 284425

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