Usage of `override` keyword.

[MikeDvorskiy]

1. Basically, 99% oneDPL code doesn't use even "virtual", because 99% of code is general template code, which provides static polymorphism.
2. override is available since C++11 , we can use override keyword. It gives more compile time coding control.
   Not using it before - it was a "gap".

Originally posted by @MikeDvorskiy in #1942 (comment)