[Remarks][NFC] Move the serialization to lib/Remarks

Separate the remark serialization to YAML from the LLVM Diagnostics.

This adds a new serialization abstraction: remarks::Serializer. It's
completely independent from lib/IR and it provides an easy way to
replace YAML by providing a new remarks::Serializer.

Differential Revision: https://reviews.llvm.org/D62632

llvm-svn: 362160