[Syntax] TypeAliasDecl's initializer is not optional

Author: DougGregor

utils/gyb_syntax_support/DeclNodes.py

@@ -25,8 +25,7 @@
              Child('Identifier', kind='IdentifierToken'),
              Child('GenericParameterClause', kind='GenericParameterClause',
                    is_optional=True),
-             Child('Initializer', kind='TypeInitializerClause',
-                   is_optional=True),
+             Child('Initializer', kind='TypeInitializerClause'),
              Child('GenericWhereClause', kind='GenericWhereClause',
                    is_optional=True),
          ]),