swift234

Refactor the handling of *ApplicationMain classes.

Rather than keeping just a "main class" in every module, track the "main file"
that's responsible for producing the module's entry point. This covers both
main source files and files containing classes marked @UIApplicationMain or
@NSApplicationMain.

This should have no functionality change, but is preparation for the next
commit, where we will preserve some of this information in serialization.

Swift SVN r24529