`type_check_only_generic` decorator

stuff like `attrgetter` aren't actually generic in real life

Start by locating the generic type-checking and decorator handling in basedmypy, then inspect how attrgetter is currently classified. Clarify the intended behavior for a type_check_only_generic decorator before changing it, and add coverage showing that the affected constructs are treated as generic only during type checking.

Written by the indexing model from the issue text.