[libc++] Forward ranges::sort to instantiations in the dylib

This patch removes `_WrapAlgPolicy` and related functionality. Instead, we explicitly forward to `__sort` now if we have an instantiation inside the dylib. If we don't we just call `__introsort`.

Reviewed By: ldionne, #libc

Spies: sstefan1, libcxx-commits

Differential Revision: https://reviews.llvm.org/D140824