Tool to produce required dependencies

[migueldeicaza]

On non-Mac systems that do not bundle Swift as part of the system, we have some guidance that says "Just copy all of these Swift libraries to your target", which includes a ton of stuff that users might not ever use. But better safe than sorry.

We should build a tool that scans the generated object files and determines which functionality is required, rather than telling everyone to distribute everything, specially for newcomers to Swift that have no appetite to figure this out.