As discussed in: http://lists.llvm.org/pipermail/llvm-dev/2018-May/123182.html
Let's start migrating to C++14. As a first step, update the GCC and LLVM
versions to the ones that first got full C++14 support, and see if bots fall
over and die. If that sticks, let's then update the rest of this file (MSVC and
standard libraries). If it all works out (let's be honest: it won't for a while)
then we can bump to -std=c++14.