Drop Visual Studio 2015 and use C++17

[wraitii]

See also VS13 ticket: #5379

To compile SM68 (#5860), we need Visual Studio 2017. SM60 is compatible with both VS15 and VS17, so it's a good idea to change then.

To compile SM78 (#5861), we need C++17, which is supported in VS17 (but other compilers might have varying support). C++17 also has minor code incompatibilities with C++11.

Note that I'm unsure if SM60 and SM68 can be compiled with c++17, which means this would be a two-stepper.

TODO:

• Update the following wiki pages: BuildInstructions, CppSupport, CodeQuality, ReviewingPatches, JenkinsSetup

[Vladislav Belov]

We have compilers on other platforms too (that might not support C++17).

[wraitii]

C++17 support is most likely good enough from Clang 5+, GCC 7+ and VS17+

We won't have much choice but to drop support for earlier versions.

[Vladislav Belov]

It's possible to mix C++ standards (with some kind of work), but we need to decide what to drop.