YAMLIO: Allow scalars to dictate quotation rules

Introduce ScalarTraits::mustQuote which determines whether or not a
StringRef needs quoting before it is acceptable to output.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205955 91177308-0d34-0410-b5e6-96231b3b80d8

docs/YamlIO.rst

@@ -426,8 +426,10 @@ looks like:
       static StringRef input(StringRef scalar, T &value) {
         // do custom parsing here.  Return the empty string on success,
         // or an error message on failure.
-        return StringRef(); 
+        return StringRef();
       }
+      // Determine if this scalar needs quotes.
+      static bool mustQuote(StringRef) { return true; }
     };