Reland: [Remarks] Support parsing remark metadata in the YAML remark parser

This adds support to the yaml remark parser to be able to parse remarks
directly from the metadata.

This supports parsing separate metadata and following the external file
with the associated metadata, and also a standalone file containing
metadata + remarks all together.

Original llvm-svn: 367148
Revert llvm-svn: 367151

This has a fix for gcc builds.

llvm-svn: 367155