Move lookupQualified from Module to DeclContext.

Qualified lookup depends not only on the current module but actually the
current file, since imports are file-scoped by default. Currently this only
affects lookup into other modules (e.g. "Foundation.NSString"), but this
could also be used for extension filtering.

This breaks one name resolution test, but the refactoring in the next
commit changes everything anyway and will restore the test.

Swift SVN r10831

lib/AST/Module.cpp

@@ -1114,6 +1114,20 @@ void Module::forAllVisibleModules(Optional<AccessPathTy> thisPath,
   forAllImportedModules<true>(this, thisPath, fn);
 }
 
+void
+FileUnit::forAllVisibleModules(std::function<bool(Module::ImportedModule)> fn) {
+  getParentModule()->forAllVisibleModules(Module::AccessPathTy(), fn);
+
+  if (auto SF = dyn_cast<SourceFile>(this)) {
+    // Handle privately visible modules as well.
+    for (auto importPair : SF->getImports()) {
+      if (!importPair.second)
+        importPair.first.second->forAllVisibleModules(importPair.first.first,
+                                                      fn);
+    }
+  }
+}
+
 void Module::collectLinkLibraries(LinkLibraryCallback callback) {
   forAllImportedModules<false>(this, AccessPathTy(),
                                [=](ImportedModule import) -> bool {