Fix TypeIs negative narrowing of union of generics

[brianschubert]

Fixes #18009, fixes #19282, fixes #17181

Modelling the runtime behavior of isinstance (which erases generic type arguments) isn't applicable to TypeIs. This PR adds a flag so that we can skip that logic deep inside conditional_types_with_intersection.

It's a little awkward having to pass a flag down through so many call levels, but I can't think of a better way.

[sterliakov]

More issues might be closed by this PR: sterliakov/mypy-issues#70 I didn't look deeper, but all four look related enough

[hauntsaninja]

Thanks for the PR! I made some small additions. We have a new hit in starlette, but I think this is correct?

Edit: see also #19320

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pytest (https://github.com/pytest-dev/pytest)
+ testing/test_monkeypatch.py:420: error: Unused "type: ignore" comment  [unused-ignore]

starlette (https://github.com/encode/starlette)
+ starlette/middleware/errors.py:178: error: Argument 1 to "run_in_threadpool" has incompatible type "Callable[[Request, Exception], Response | Awaitable[Response]] | Callable[[WebSocket, Exception], Awaitable[None]]"; expected "Callable[[Request, Exception], Response]"  [arg-type]
+ starlette/_exception_handler.py:61: error: Argument 1 to "run_in_threadpool" has incompatible type "Callable[[Request, Exception], Response | Awaitable[Response]] | Callable[[WebSocket, Exception], Awaitable[None]]"; expected "Callable[[Request | WebSocket, Exception], Any]"  [arg-type]
+ starlette/routing.py:68: error: Incompatible types in assignment (expression has type "Callable[..., Awaitable[Any]] | partial[Coroutine[Any, Any, Awaitable[Response] | Response]]", variable has type "Callable[[Request], Awaitable[Response]]")  [assignment]
+ starlette/routing.py:68: error: Too few arguments for "run_in_threadpool"  [call-arg]