Added executable entry-point via @main type.

When a type (class, enum, or struct) is annotated @main, it is required
to provide a function with the following signature:

  static func main() -> ()

That function will be called when the executable the type is defined
within is launched.

lib/Serialization/Serialization.cpp

@@ -4986,7 +4986,7 @@ void Serializer::writeAST(ModuleOrSourceFile DC) {
   }
   for (auto nextFile : files) {
     if (nextFile->hasEntryPoint())
-      entryPointClassID = addDeclRef(nextFile->getMainClass());
+      entryPointClassID = addDeclRef(nextFile->getMainDecl());
 
     // FIXME: Switch to a visitor interface?
     SmallVector<Decl *, 32> fileDecls;