[DiagnosticInfo] Add support for preserving newlines in remark arguments.
This patch adds a new type StringBlockVal which can be used to emit a
YAML block scalar, which preserves newlines in a multiline string. It
also updates MappingTraits<DiagnosticInfoOptimizationBase::Argument> to
use it for argument values with more than a single newline.
This is helpful for remarks that want to display more in-depth
information in a more structured way.
Reviewers: thegameg, anemet
Reviewed By: anemet
Subscribers: hfinkel, hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D57159