Add more override keywords

Mark more functions with "override" keyword, just like
it was done in commit 2460f97152.

This should prevent compiler from complaining while compiling both
user code, and the googletest code itself with the -Wsuggest-override
option turned on; with the exception of:
 * calls to new MOCK_METHOD() in test/gmock-function-mocker_test.cc
 * calls to old MOCK_METHODx()/MOCK_CONST_METHODx() in other
   unit test files.

Closes #2493

googlemock/include/gmock/gmock-generated-matchers.h.pump

@@ -302,13 +302,13 @@ $var param_field_decls2 = [[$for j
      public:\
       [[$if i==1 [[explicit ]]]]gmock_Impl($impl_ctor_param_list)\
           $impl_inits {}\
-      virtual bool MatchAndExplain(\
+      bool MatchAndExplain(\
           GTEST_REFERENCE_TO_CONST_(arg_type) arg,\
-          ::testing::MatchResultListener* result_listener) const;\
-      virtual void DescribeTo(::std::ostream* gmock_os) const {\
+          ::testing::MatchResultListener* result_listener) const override;\
+      void DescribeTo(::std::ostream* gmock_os) const override {\
         *gmock_os << FormatDescription(false);\
       }\
-      virtual void DescribeNegationTo(::std::ostream* gmock_os) const {\
+      void DescribeNegationTo(::std::ostream* gmock_os) const override {\
         *gmock_os << FormatDescription(true);\
       }\$param_field_decls
      private:\