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/Serialization/ModuleFile.h

@@ -61,7 +61,10 @@ enum class ModuleStatus {
 /// A serialized module, along with the tools to access it.
 class ModuleFile {
   /// A reference back to the AST representation of the module.
-  Module *ModuleContext;
+  Module *ModuleContext = nullptr;
+
+  /// The module shadowed by this module, if any.
+  Module *ShadowedModule = nullptr;
 
   /// The module file data.
   llvm::OwningPtr<llvm::MemoryBuffer> InputFile;
@@ -246,6 +249,12 @@ private:
   /// Returns the identifier with the given ID, deserializing it if needed.
   Identifier getIdentifier(serialization::IdentifierID IID);
 
+  /// Returns the appropriate module for the given name.
+  ///
+  /// If the name matches the name of the current module, a shadowed module
+  /// is loaded instead. An empty name represents the Builtin module.
+  Module *getModule(Identifier name);
+
   /// Populates TopLevelIDs for name lookup.
   void buildTopLevelDeclMap();
 
@@ -291,6 +300,9 @@ public:
   ///
   /// If none is found, returns null.
   OperatorDecl *lookupOperator(Identifier name, DeclKind fixity);
+
+  /// Adds any reexported modules to the given vector.
+  void getReexportedModules(SmallVectorImpl<Module*> &results);
 };
 
 } // end namespace swift