Using the new ObjC type analyzer API, scan the bound packages for
references to ObjC classes and protocols and generate Go wrappers for them.
This is the second part of the implementation of proposal golang/go#17102.
For golang/go#17102
Change-Id: I773db7b0362a7ff526d0a0fd6da5b2fa33301144
Reviewed-on: https://go-review.googlesource.com/29174
Reviewed-by: David Crawshaw <crawshaw@golang.org>
