New semantic analyzer: invalid type variable produces additional error

[ilevkivskyi]

This test case fails:

[case testMoreInvalidTypevarArguments]
from typing import TypeVar

defer: Yes

S = TypeVar('S', covariant=True, contravariant=True)  # E: TypeVar cannot be both covariant and contravariant

class Yes: ...
[builtins fixtures/bool.pyi]

because there is an additional (and somewhat confusing) error:

Cannot redefine 'S' as a type variable

See also #6300 and #6563 (ideally, the latter should also fix this issue).

[ilevkivskyi]

This is quite annoying when testing (namely when running tests with an extra iteration), so raising priority to normal.