Add text_choices for isolated and _const token in FunctionParameter

This would have caught a misparsing issue that I’m fixing in the old libSyntax parser.

Author: ahoppen

gyb_syntax_support/DeclNodes.py

@@ -377,9 +377,9 @@
          children=[
              Child('Attributes', kind='AttributeList', name_for_diagnostics='attributes',
                    collection_element_name='Attribute', is_optional=True),
-             Child('IsolatedToken', kind='Token',
+             Child('IsolatedToken', kind='IdentifierToken', text_choices=['isolated'],
                    is_optional=True),
-             Child('ConstToken', kind='Token',
+             Child('ConstToken', kind='IdentifierToken', text_choices=['_const'],
                    is_optional=True),
              Child('FirstName', kind='Token', name_for_diagnostics='name',
                    token_choices=[