swift234

Clang importer: filter out hidden Clang entries from name lookup.

The Swift name lookup tables record name -> declaration mappings
without respect to submodule visibility. When we do name lookup via
those lookup tables, filter out declarations that are not visible.

Note that this implementation is broken in the same way that the
pre-name-lookup-tables implementation is broken, because it uses
Clang's declaration visibility indicators rather than filtering out
based on submodule lookups. We should fix this behavior to provide
proper submodule lookup, but not until after we've matched the
existing behavior with Swift's name lookup tables.