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.

llvm-svn: 132570
diff --git a/clang/lib/Frontend/Warnings.cpp b/clang/lib/Frontend/Warnings.cpp
index 829ac9e..f12b484 100644
--- a/clang/lib/Frontend/Warnings.cpp
+++ b/clang/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());
   }
 }