Add many missing override keywords

This helps corrects builds with -Werror=suggest-overrides

googletest/include/gtest/gtest-matchers.h

@@ -474,13 +474,13 @@ class PolymorphicMatcher {
    public:
     explicit MonomorphicImpl(const Impl& impl) : impl_(impl) {}
 
-    virtual void DescribeTo(::std::ostream* os) const { impl_.DescribeTo(os); }
+    void DescribeTo(::std::ostream* os) const override { impl_.DescribeTo(os); }
 
-    virtual void DescribeNegationTo(::std::ostream* os) const {
+    void DescribeNegationTo(::std::ostream* os) const override {
       impl_.DescribeNegationTo(os);
     }
 
-    virtual bool MatchAndExplain(T x, MatchResultListener* listener) const {
+    bool MatchAndExplain(T x, MatchResultListener* listener) const override {
       return impl_.MatchAndExplain(x, listener);
     }
 

googletest/include/gtest/gtest-param-test.h

@@ -416,7 +416,7 @@ internal::CartesianProductHolder<Generator...> Combine(const Generator&... g) {
       : public test_suite_name {                                               \
    public:                                                                     \
     GTEST_TEST_CLASS_NAME_(test_suite_name, test_name)() {}                    \
-    virtual void TestBody();                                                   \
+    void TestBody() override;                                                  \
                                                                                \
    private:                                                                    \
     static int AddToRegistry() {                                               \

googletest/include/gtest/internal/gtest-internal.h

@@ -1354,7 +1354,7 @@ constexpr bool InstantiateTypedTestCase_P_IsDeprecated() { return true; }
     GTEST_TEST_CLASS_NAME_(test_suite_name, test_name)() {}                   \
                                                                               \
    private:                                                                   \
-    virtual void TestBody();                                                  \
+    void TestBody() override;                                                 \
     static ::testing::TestInfo* const test_info_ GTEST_ATTRIBUTE_UNUSED_;     \
     GTEST_DISALLOW_COPY_AND_ASSIGN_(GTEST_TEST_CLASS_NAME_(test_suite_name,   \
                                                            test_name));       \

googletest/include/gtest/internal/gtest-port.h

@@ -1808,7 +1808,7 @@ class GTEST_API_ ThreadLocal {
   class DefaultValueHolderFactory : public ValueHolderFactory {
    public:
     DefaultValueHolderFactory() {}
-    virtual ValueHolder* MakeNewHolder() const { return new ValueHolder(); }
+    ValueHolder* MakeNewHolder() const override { return new ValueHolder(); }
 
    private:
     GTEST_DISALLOW_COPY_AND_ASSIGN_(DefaultValueHolderFactory);
@@ -1817,7 +1817,7 @@ class GTEST_API_ ThreadLocal {
   class InstanceValueHolderFactory : public ValueHolderFactory {
    public:
     explicit InstanceValueHolderFactory(const T& value) : value_(value) {}
-    virtual ValueHolder* MakeNewHolder() const {
+    ValueHolder* MakeNewHolder() const override {
       return new ValueHolder(value_);
     }