swift234

Replace ASTMutationListener with a new LazyResolver callback.

..."resolveExternalDeclImplicitMembers".

Now that the ClangImporter has direct access to the type-checker (through
a LazyResolver), there's no reason to bounce through an obtusely generic
interface on ASTContext. Just call through directly to handle the implicit
members and conformances of external decls.

There's no actual functionality change here, though we can probably do
further cleanup in this area.

Swift SVN r15356