[SyntaxParse] Refactor generic requirement syntax structure

Re-apply a part of 0569cbfb28 after
reverting ASTGen changes. This is still an improvement.

utils/gyb_syntax_support/NodeSerializationCodes.py

@@ -233,6 +233,7 @@ SYNTAX_NODE_SERIALIZATION_CODES = {
     'PoundAssertStmt': 229,
     'SomeType': 230,
     'CustomAttribute': 231,
+    'GenericRequirement': 232,
 }