Reland "[Remarks] Refactor remark diagnostic emission in a RemarkStreamer"

This allows us to store more info about where we're emitting the remarks
without cluttering LLVMContext. This is needed for future support for
the remark section.

Differential Revision: https://reviews.llvm.org/D58996

Original llvm-svn: 355507

llvm-svn: 355514