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f5096f20 - [parser] Flag single unparenthesized generator expr with trailing comma in arguments. (#17893)

[parser] Flag single unparenthesized generator expr with trailing comma in arguments. (#17893) Fixes #17867 ## Summary The CPython parser does not allow generator expressions which are the sole arguments in an argument list to have a trailing comma. With this change, we start flagging such instances. ## Test Plan Added new inline tests.