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

lib/AST/Module.cpp

@@ -343,11 +343,7 @@ namespace {
   };
 }
 
-/// Returns true if the two access paths contain the same chain of identifiers.
-///
-/// Source locations are ignored here.
-static bool isSameAccessPath(Module::AccessPathTy lhs,
-                             Module::AccessPathTy rhs) {
+bool Module::isSameAccessPath(AccessPathTy lhs, AccessPathTy rhs) {
   using AccessPathElem = std::pair<Identifier, SourceLoc>;
   if (lhs.size() != rhs.size())
     return false;