Handle TypeVarTupleType when checking overload constraints (#16428)
Fixes https://github.com/python/mypy/issues/16427
The test case added in the first commit crashes.
The second commit addresses the crash - I don't know whether this fix is
correct, it just happens to stop the crash but it leads to a code branch
which just `continue`s out of a for loop iteration, so it might be
bypassing something it shouldn't. I don't completely understand it.
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Co-authored-by: Ivan Levkivskyi <levkivskyi@gmail.com>