[libc++] Build the dylib with C++17 to allow aligned new/delete

This allows simplifying the implementation of barriers.

This is a re-commit of 1ac403bd, which had to be reverted in
64a9c944 because the minimum CMake version wasn't high enough.
Now that we've upgraded, we can do this.

Differential Revision: https://reviews.llvm.org/D75243