The lit feature `objc_interop` is used as a proxy for the availability
of libdispatch and Foundation.
Although we don't support Obj-C on non-Apple platforms, we support
libdispatch and Foundation. Let's break up `objc_interop` into
finer-grained categories so we can run more tests on non-Apple
platforms.
This patch adds lit features `libdispatch` and `foundation` (currently
only enabled on Apple platforms) and removes the `objc_interop`
dependency from libdispatch tests.
