Due to a bug, -Wno-everything works like -Weverything. Fix the bug by having
-Wno-everything remap all warnings to ignored.

We can now use "-Wno-everything -W<warning>" to ignore all warnings except
specific ones.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149121 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Frontend/Warnings.cpp b/lib/Frontend/Warnings.cpp
index 576dd3d..f68e03b 100644
--- a/lib/Frontend/Warnings.cpp
+++ b/lib/Frontend/Warnings.cpp
@@ -110,8 +110,14 @@
       // -Weverything is a special case as well.  It implicitly enables all
       // warnings, including ones not explicitly in a warning group.
       if (Opt == "everything") {
-        if (SetDiagnostic)
-          Diags.setEnableAllWarnings(true);
+        if (SetDiagnostic) {
+          if (isPositive) {
+            Diags.setEnableAllWarnings(true);
+          } else {
+            Diags.setEnableAllWarnings(false);
+            Diags.setMappingToAllDiagnostics(diag::MAP_IGNORE);
+          }
+        }
         continue;
       }