Don't warn about -Wno-foo where foo is an unknown warning option. Explicitly
passing -Wunknown-warning-option will re-enable warnings about -Wno-foo.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@132570 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/Warnings.cpp b/lib/Frontend/Warnings.cpp
index 829ac9e..f12b484 100644
--- a/lib/Frontend/Warnings.cpp
+++ b/lib/Frontend/Warnings.cpp
@@ -123,6 +123,8 @@
     }
 
     if (Diags.setDiagnosticGroupMapping(Opt, Mapping))
-      Diags.Report(diag::warn_unknown_warning_option) << ("-W" + Opt.str());
+      Diags.Report(isPositive ? diag::warn_unknown_warning_option :
+                   diag::warn_unknown_negative_warning_option)
+          << ("-W" + Opt.str());
   }
 }