Revert "[clang][Darwin] Always set the default C++ Standard Library to libc++"

This reverts commit c5ccb78a.

We're seeing darwin-stdlib.cpp fail on our linux, mac, and windows
builders:
https://luci-milo.appspot.com/ui/p/fuchsia/builders/toolchain.ci/clang-linux-x64/b8806821020552676065/overview