[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

llvm-svn: 352216