[Support] Make support types more easily printable.

Summary:
Error's new operator<< is the first way to print an error without consuming it.

formatv() can now print objects with an operator<< that works with raw_ostream.

Reviewers: bkramer

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D48966

llvm-svn: 336412