[ty] Ensure Literal types are considered assignable to anything their Instance supertypes are assignable to#18351

AlexWaygood merged 5 commits intoastral-sh:mainastral-sh/ruff:mainfrom

MatthewMckee4:literal-assignability-with-nominal-instanceMatthewMckee4/ruff:literal-assignability-with-nominal-instanceCopy head branch name to clipboard