[Remarks] Pass StringBlockValue as StringRef.

[fhahn]

Summary

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.

[thegameg]

LGTM, thanks.

llvm/lib/Remarks/YAMLRemarkSerializer.cpp

template <> struct MappingTraits<RemarkLocation> {
 
   static const bool flow = true;
 };
 
 /// Helper struct for multiline string block literals. Use this type to preserve
 /// newlines in strings.
 struct StringBlockVal {
   StringRef Value;
-  StringBlockVal(const std::string &Value) : Value(Value) {}
+  StringBlockVal(StringRef R) : Value(R) {}
 };
 
 template <> struct BlockScalarTraits<StringBlockVal> {
   static void output(const StringBlockVal &S, void *Ctx, raw_ostream &OS) {
     return ScalarTraits<StringRef>::output(S.Value, Ctx, OS);
   }
 
   static StringRef input(StringRef Scalar, void *Ctx, StringBlockVal &S) {