One interesting thing here is that the decision of whether or not to
track system dependencies ends up going into the cache key for a
swiftmodule built from a parseable interface, because it affects that
module's up-to-date check. If we didn't include
-track-system-dependencies in the cache key, we could end up tracking
system dependencies for some modules but not others in the same build.
There's a bit of a bug here where they're /not/ honored if the
top-level invocation isn't tracking /any/ dependencies, but given
how uncommon this flag is in practice that's probably okay for now.
Still TODO: honor this for -build-swiftmodule-from-parseable-interface
as well. That's currently not tracking dependencies at all and it
probably should.
