[OPENMP]Add assignment operator in UDR test, NFC.

Add assignment operator in the test to check that even if the operator
was declare explicitly, the constructor is called in the user-defined
reduction initializer anyway.

clang/test/OpenMP/for_reduction_codegen_UDR.cpp

@@ -38,6 +38,7 @@ struct S : public BaseS, public BaseS1 {
   T f;
   S(T a) : f(a + g) {}
   S() : f(g) {}
+  S& operator=(const S&);
   ~S() {}
 };
 void red(BaseS1&, const BaseS1&);