[modules] Introduce Module::getTopLevelDecls() to get the local-in-module top-level decls.

getDisplayDecls() was introduced for ":print_module" and works slightly differently, e.g.
it will return the decls from a shadowed clang module, since we want to display them.

Swift SVN r7909

lib/Serialization/ModuleFile.h

@@ -363,9 +363,14 @@ public:
   /// Reports all link-time dependencies.
   void getLinkLibraries(Module::LinkLibraryCallback callback) const;
   
+  /// Adds all top-level decls to the given vector.
+  void getTopLevelDecls(SmallVectorImpl<Decl*> &Results);
+
   /// Adds all top-level decls to the given vector.
   ///
   /// This includes all decls that should be displayed to clients of the module.
+  /// This can differ from \c getTopLevelDecls, e.g. it returns decls from a
+  /// shadowed clang module.
   void getDisplayDecls(SmallVectorImpl<Decl*> &results);
 
   StringRef getModuleFilename() const {