Use a normal error when the new parser's consistency checking fails

Author: DougGregor

include/swift/AST/DiagnosticsParse.def

@@ -1962,6 +1962,12 @@ ERROR(attr_requires_concurrency, none,
       "concurrency is enabled",
       (StringRef, bool))
 
+//------------------------------------------------------------------------------
+// MARK: Consistency diagnostics
+//------------------------------------------------------------------------------
+ERROR(new_parser_failure, none,
+      "new parser has failed consistency checking; please see errors above", ())
+
 //------------------------------------------------------------------------------
 // MARK: syntax parsing diagnostics
 //------------------------------------------------------------------------------

lib/Parse/ParseRequests.cpp

@@ -219,9 +219,8 @@ SourceFileParsingResult ParseSourceFileRequest::evaluate(Evaluator &evaluator,
           bufferRange.str().data(), bufferRange.getByteLength(),
           SF->getFilename().str().c_str(), flags);
 
-      // FIXME: Produce an error on round-trip failure.
       if (roundTripResult)
-        abort();
+        ctx.Diags.diagnose(SourceLoc(), diag::new_parser_failure);
     }
   }
 #endif