Dependency tracking for cached compiled modules (compiled from
swiftinterfaces) can lead to a high percentage of the module being
SDK-relative paths when -track-system-dependencies is on. Cut down on
this by storing directory names in a separate record that gets
referenced from each file dependency. (Since a lot of per-file
dependencies are header files in a common directory, this is a win.)
We can do something more clever in the future, but this is a
reasonable start for, say, the overlays.
rdar://problem/50449802
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.
