[swiftc] Add completion spec

Sanity checks

• My issue relates to a specific CLI completion spec (e.g. git checkout is missing options in git completion spec). If your issue is more general, please create your issue here: withfig/fig
• I have searched github.com/withfig/autocomplete/issues and there are no duplicates of my issue
• I would like to work on this.

What CLI tool does this relate to?

swiftc

Which statement makes the most sense?

This completion spec does not exist

Issue Details

Add support for the Swift compiler.