std::result_of is removed in C++20

std::result_of is removed in C++20, causing gmock-actions.h:819 to fail in Visual Studio 2019 16.6.2 with C++20 enabled. However std::invoke_result is available since C++17.

For C++20:
```
  using ReturnType = typename std::result_of<MethodPtr(Class*)>::type;
```
should be replaced with:
```
  using ReturnType = typename std::invoke_result_t<MethodPtr, Class*>;
```