[libc++] Drop support for CRTs older than VS 2015

LLVM dropped support for Visual Studio versions older than 2015 quite
some time ago, so I consider it safe to drop libc++'s support for older
CRTs. The CRT in Visual Studio 2015 provides a lot of previously missing
functions, so targeting it requires less special casing.

Differential Revision: https://reviews.llvm.org/D31798

llvm-svn: 299743