[libc++] Remove unnecessary MSVCRT exclusions

Visual Studio 2013 and up have these functions, and we don't need to
support older versions.

There are some remaining _LIBCPP_MSVCRT exclusions which are present on
Visual Studio 2015 but not 2013. Those will be addressed in a follow-up.

Differential Revision: https://reviews.llvm.org/D26377

llvm-svn: 286202