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
b3c5a68acf