[C++11] Remove an LLVM_OVERRIDE use that I missed in my previous commit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202630 91177308-0d34-0410-b5e6-96231b3b80d8

lib/ASTMatchers/Dynamic/Marshallers.h

@@ -637,7 +637,7 @@ public:
       *LeastDerivedKind = Kind;
     return true;
   }
-  bool isPolymorphic() const LLVM_OVERRIDE { return true; }
+  bool isPolymorphic() const override { return true; }
 
 private:
   const unsigned MinCount;