Incremental swift compilation

[keith]

Through some cursory tests to integrate bazel into our all swift project, we noticed that bazel doesn't currently support incremental swift compilation.

We've taken a stab at adding this by adding the -incremental and -emit-dependencies compiler flags and adding the dependency information to the output_file_map.json.

If I then run the commands that are being ran by bazel manually, I get incremental compilation, but running from bazel, it does not work incrementally. It seems as though bazel is removing the previously generated files after each change.

Is there a reason the initial Swift support didn't support incremental compilation? Also are there any plans to add this? I would be happy to submit a PR for the changes we've made if you can point me in the right direction on the problems we've ran in to.

Thanks!

[keith]

Closing for bazelbuild/rules_apple#21