[F812] Warn when list comprehension redefines a name

[ansley]

Stack from ghstack (oldest at bottom):

• #277
• -> #276
• #275
• #274
• #273
• #272
• #271

This lint rule is probably in Flake8 to prevent elements from leaking out of the comprehension scope, but the behavior was fixed in Python3. I worry such a strict lint rule will cause more problems than it solves, considering we use Python 3.10 at Meta. Perhaps we should consider disabling this rule by default.