[Remarks] Pass StringBlockValue as StringRef.

After changing the remark serialization, we now pass StringRefs to the
serializer. We should use StringRef for StringBlockVal, to avoid
creating temporary objects, which then cause StringBlockVal.Value to
point to invalid memory.

Reviewers: thegameg, anemet

Reviewed By: thegameg

Differential Revision: https://reviews.llvm.org/D68571

llvm-svn: 373923