[Frontend] Add a more descriptive error for a TBD validation failure.

Author: huonw

include/swift/AST/DiagnosticsFrontend.def

@@ -220,6 +220,10 @@ ERROR(symbol_in_ir_not_in_tbd,none,
       "symbol '%0' (%1) is in generated IR file, but not in TBD file",
       (StringRef, StringRef))
 
+ERROR(tbd_validation_failure,none,
+      "please file a radar or open a bug on bugs.swift.org with this code, and "
+      "add -Xfrontend -validate-tbd-against-ir=none to squash the errors", ())
+
 ERROR(redundant_prefix_compilation_flag,none,
       "invalid argument '-D%0'; did you provide a redundant '-D' in your build settings?", 
       (StringRef))    

lib/FrontendTool/TBD.cpp

@@ -113,6 +113,10 @@ static bool validateSymbolSet(DiagnosticEngine &diags,
     }
   }
 
+  if (error) {
+    diags.diagnose(SourceLoc(), diag::tbd_validation_failure);
+  }
+
   return error;
 }