[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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@286202 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed