Don't #define _LIBCPP_HAS_NO_MONOTONIC_CLOCK on __APPLE__

This fixes PR20839, which was a bug in r216949.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@216975 91177308-0d34-0410-b5e6-96231b3b80d8
Silence _LIBCPP_ELAST porting warning on __APPLE__

This fixes a warning accidentally introduced in r216943.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@216977 91177308-0d34-0410-b5e6-96231b3b80d8