Warn on usages of `Literal[None]`

This is redundant and confusing and would benefit from some errors.

Start by locating the code that analyzes typing.Literal expressions and the existing tests for emitted errors or warnings. Determine how Literal[None] is currently represented and where diagnostics are asserted. Done means usages of Literal[None] produce the intended warning without changing other Literal behavior.

Written by the indexing model from the issue text.