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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@367148 91177308-0d34-0410-b5e6-96231b3b80d8