swift234

Introduce Module::forAllVisibleModules, and use it where useful.

This iterates over a module's exports, transitively, in an unspecified
but deterministic order. This is useful for any sort of lookup and for
managing transitive inclusion. It also allows us to remove the hack in
Sema for loading a Clang module's adapter module, and just rely on the
previous commit.

Swift SVN r6699