Move to C++14

Summary:
I just bumped the minimum compiler versions to support C++14 in D66188.

Following [our process](http://llvm.org/docs/DeveloperPolicy.html#toolchain) and [our previous agreement](http://lists.llvm.org/pipermail/llvm-dev/2019-January/129452.html), I'm now officially bumping the C++ version to 14 and updating the documentation.

Subscribers: mgorny, jkorous, dexonsmith, llvm-commits, chandlerc, thakis, EricWF, jyknight, lhames, JDevlieghere

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66195

llvm-svn: 368887