swift234

Always rely on Clang lookups, not Swift lookups, to establish method overrides for imported methods.

r4035 introduced overridden-method searches into the Clang importer by
looking into the Swift declarations. However, because Objective-C
allows forward declarations, there's no order in which we could
populate the Swift declarations to ensure that all of the appropriate
overrides were found. Instead, we do the right (and lazy) thing of
performing the lookup within Clang, then importing the results. This
change also introduces caching for mirrored protocol imports, so they
can be imported lazily.

Fixes <rdar://problem/13219096>.



Swift SVN r4072