swift234

Implement proper shadowing rules for qualified lookup.

This required a general reworking of the algorithm for qualified name lookup.
Originally, qualified lookup only applied to a module -- not to its exports.
This meant that "Cocoa.NSWindow" would fail, so that was changed to grovel
through all exported modules looking for decls if the top-level module didn't
provide any.

Now, we actually do a breadth-based search, stopping at each level if decls
are provided for a given name. We also now prefer scoped imports to unscoped
imports, so "import abcde" and "import struct asdf.D" will result in a
(qualified) reference to 'D' being unambiguous.

Not working yet:
 - Shadowing for unqualified lookup.
 - Shadowing by types, so that overloads from this module can merge with
   overloads from its exports.

Swift SVN r7014