Enable valid-parse checking with the new parser

Ensure that any time the existing parser accepts a source file,
the new parser produces a valid parse (no unexpected/missing nodes) for
that same source file.

Author: DougGregor

lib/Basic/LangOptions.cpp

@@ -33,6 +33,7 @@ LangOptions::LangOptions() {
   // Note: Introduce default-on language options here.
 #ifndef NDEBUG
   Features.insert(Feature::ParserRoundTrip);
+  Features.insert(Feature::ParserValidation);
 #endif
 }