Fix#14558
## Summary
- Add `typing.NoReturn` and `typing.Never` to known instances and infer
them as `Type::Never`
- Add `is_assignable_to` cases for `Type::Never`
I skipped emitting diagnostic for when a function is annotated as
`NoReturn` but it actually returns.
## Test Plan
Added tests from
https://github.com/python/typing/blob/main/conformance/tests/specialtypes_never.py
except from generics and checking if the return value of the function
and the annotations match.
---------
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
Co-authored-by: Carl Meyer <carl@astral.sh>
## Summary
Closes#14588
```py
x: Literal[42, "hello"] = 42 if bool_instance() else "hello"
reveal_type(x) # revealed: Literal[42] | Literal["hello"]
_ = ... if isinstance(x, str) else ...
# The `isinstance` test incorrectly narrows the type of `x`.
# As a result, `x` is revealed as Literal["hello"], but it should remain Literal[42, "hello"].
reveal_type(x) # revealed: Literal["hello"]
```
## Test Plan
mdtest included!
---------
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
