[red-knot] support for typing.reveal_type (#13384)
Add support for the `typing.reveal_type` function, emitting a diagnostic
revealing the type of its single argument. This is a necessary piece for
the planned testing framework.
This puts the cart slightly in front of the horse, in that we don't yet
have proper support for validating call signatures / argument types. But
it's easy to do just enough to make `reveal_type` work.
This PR includes support for calling union types (this is necessary
because we don't yet support `sys.version_info` checks, so
`typing.reveal_type` itself is a union type), plus some nice
consolidated error messages for calls to unions where some elements are
not callable. This is mostly to demonstrate the flexibility in
diagnostics that we get from the `CallOutcome` enum.
