build: set _LIBCPP_DEBUG=1 in depends when debugging for darwin

[fanquake]

#21869 added -D_LIBCPP_DEBUG=1 to the Linux CPPFLAGS in depends. Ideally we could add this to the darwin CPPFLAGS in depends as well, however that does not work out of the box.

Related: https://libcxx.llvm.org/docs/DesignDocs/DebugMode.html.

[fanquake]

Closing for now, as the way debugging is enabled in LLVM is/has changed, and I still don't think it'll be so straight forward for us to do this for macOS.