Emit a warning when instrumentation encounters a type it cannot check

### Things to check first

- [X] I have searched the existing issues and didn't find my feature already requested there

### Feature description

The instrumentation code should emit an `InstrumentationWarning` (when in debug mode) when it encounters a type that has been designated as "ignored" (due to, for example, being imported under `if TYPE_CHECKING:`.

### Use case

It may be sometimes difficult to understand why a type check has been omitted from the instrumented code. Adding code comments for this is not possible because comments are not part of the abstract syntax tree.