Fix Clang-Tidy warnings related to modernize-use-override

[SunBlack]

Currently we have the problem that we have the static code analysis of MSVC enabled, but some warnings cannot be marked as errors, because they come into our code via macros, where the definition of external/system includes does not help. One issue is C26435 (Use single virtual specification).

For this MR I checked all warnings related:

• Clang-Tidy: modernize-use-override
• MSVC: C26433, C26435 & C26436

Furthermore I replaced all BENCHMARK_OVERRIDE by override within all cc files (as the readme states that C++11 is required to compile this framework and there was already sometimes used override without the macro) - the header files are still using BENCHMARK_OVERRIDE to keep C++03 compatibility.

[dmah42]

thanks!