Provide a skeleton for re-exports from serialized modules...

...and use it for shadowed modules (e.g. the Clang module "Foundation"
referenced by the Swift module "Foundation"), so that we can actually
find "NSString" when building AppKit.

Additionally, record shadowed modules as dependencies, so that they can
be loaded when the adapter module is loaded.

Swift SVN r6522

lib/AST/Module.cpp

@@ -368,6 +368,11 @@ LoadedModule::lookupOperator<InfixOperatorDecl>(Identifier name) {
   return cast_or_null<InfixOperatorDecl>(result);
 }
 
+void LoadedModule::getReexportedModules(SmallVectorImpl<Module *> &exports) {
+  auto owner = static_cast<ModuleLoader*>(LookupCachePimpl);
+  return owner->getReexportedModules(this, exports);
+}
+
 
 
 //===----------------------------------------------------------------------===//