Silence -Wimplicit-fallthrough warnings with the generated code; NFC.

llvm-svn: 316075
diff --git a/clang/utils/TableGen/ClangAttrEmitter.cpp b/clang/utils/TableGen/ClangAttrEmitter.cpp
index c3d593b..effabcc 100644
--- a/clang/utils/TableGen/ClangAttrEmitter.cpp
+++ b/clang/utils/TableGen/ClangAttrEmitter.cpp
@@ -2794,7 +2794,7 @@
       GenerateHasAttrSpellingStringSwitch(I->second, OS, Spelling, I->first);
       OS << "}";
     }
-    OS << "\n}\n";
+    OS << "\n} break;\n";
   };
   fn("CXX11", "CXX", CXX);
   fn("C2x", "C", C2x);