[libc++] Refactor the Apple build scripts

This patch upstreams some changes we've made internally to how we're
building the libc++ dylib on Apple platforms. The goal is still to
eventually get rid of `apple-install-libcxx.sh` entirely and have a
proper way to mirror what we do internally with just the normal CMake
configuration.

Differential Revision: https://reviews.llvm.org/D118912