[DiagnosticInfo] Add support for preserving newlines in remark arguments.

[fhahn]

Summary

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.

[anemet]

LGTM, thanks!

llvm/lib/IR/DiagnosticInfo.cpp

static void mapping(IO &io, DiagnosticLocation &DL) {
     io.mapRequired("File", File);
     io.mapRequired("Line", Line);
     io.mapRequired("Column", Col);
   }
 
   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) {}
+};
+
+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) {
+    return ScalarTraits<StringRef>::input(Scalar, Ctx, S.Value);
+  }
+};
+
 // Implement this as a mapping for now to get proper quotation for the value.
 template <> struct MappingTraits<DiagnosticInfoOptimizationBase::Argument> {
   static void mapping(IO &io, DiagnosticInfoOptimizationBase::Argument &A) {
     assert(io.outputting() && "input not yet implemented");
+    // Emit a string block scalar for multiline strings, to preserve newlines.
+    if (StringRef(A.Val).count('\n') > 1) {
+      StringBlockVal S(A.Val);
+      io.mapRequired(A.Key.data(), S);
+    } else
       io.mapRequired(A.Key.data(), A.Val);
     if (A.Loc.isValid())
       io.mapOptional("DebugLoc", A.Loc);
   }
 };
 
 } // end namespace yaml
 } // end namespace llvm
 
 LLVM_YAML_IS_SEQUENCE_VECTOR(DiagnosticInfoOptimizationBase::Argument)