Allow VersionPrinter to print to arbitrary raw_ostreams

Summary:
I would like to add printing of registered targets to clang's version
information.  For this to work correctly, the VersionPrinter logic in
CommandLine.cpp should support printing to arbitrary raw_ostreams,
instead of always defaulting to outs().

Add a raw_ostream& parameter to the function pointer type used for
VersionPrinter, and while doing so, introduce a typedef for convenience.

Note that VersionPrinter::print() will still default to using outs(),
the clang part will necessarily go into a separate review.

Reviewers: beanz, chandlerc, dberris, mehdi_amini, zturner

Reviewed By: mehdi_amini

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33899

llvm-svn: 304835