[llvm-dev] Retiring VS2015 Support

[Simon Pilgrim via llvm-dev]

AFAICT we traditionally only support building llvm/clang with the 2 most 
recent releases of Visual Studio.

With the release of VS2019, what do people think of retiring VS2015 
support before the next major llvm/clang release?

As well as simplifying C++14 migration (admittedly only a little), this 
should also let us remove a number of hacks (e.g. the 
alignas/AlignedCharArray hack in AlignOf.h), as well as re-enabling a 
number of warnings in HandleLLVMOptions.cmake.

I haven't noticed any build bots still referring to VS2015 (at least not 
by name).

Simon.