libSyntax: support generic parameter clause. (#13286)

This patch also performs minor refactoring to align syntax parsing
context with the right scope. We start to support the generic clauses
because they are necessary pieces to construct struct or
function syntax node.

utils/gyb_syntax_support/GenericNodes.py

@@ -35,6 +35,8 @@ GENERIC_NODES = [
     #                    | type-name : protocol-composition-type
     Node('GenericParameter', kind='Syntax',
          children=[
+             Child('Attributes', kind='AttributeList',
+                   is_optional=True),
              Child('Name', kind='IdentifierToken'),
              Child('Colon', kind='ColonToken',
                    is_optional=True),