swift234

Perform proper shadowing when looking for module-level visible decls.

This makes lookupVisibleDecls use the same code path as qualified and
unqualified lookup. This is more expensive, because it needs to actually
collect a list of /all/ decls in order to properly compute shadowing
information, but this list is not likely to change once the set of imports
has been computed, so we can cache it.

Swift SVN r7168