Fix clang `-Winconsistent-missing-override` warnings
Fix clang `-Winconsistent-missing-override` warnings

`DescribeTo(..)` and `MatchAndExplain(..)` in `gmock-matchers_test` both
override virtual methods. Remove the `virtual` keyword and apply `override` to
them instead.

Signed-off-by: Enji Cooper <d6bb01297bdcc8d1a5e14be12c1049fd9e385b00@gmail.com>

xnagireddy/googletest,xnagireddy/googletest,gianricardo/googletest,empiredan/googletest,google/googletest,jlanecox/googletest,google/googletest,google/googletest,antidotcb/googletest,empiredan/googletest,empiredan/googletest,jlanecox/googletest,jlanecox/googletest,jlanecox/googletest,gianricardo/googletest,antidotcb/googletest,gianricardo/googletest,xnagireddy/googletest,google/googletest,antidotcb/googletest,xnagireddy/googletest,antidotcb/googletest