NilAway reports guard missing for errors checked by switch

[yuxincs]

For error-returning functions, NilAway will check if the error is properly checked, otherwise the pointer result will be treated as nilable. However, NilAway currently only supports checking the errors via an if statement, but not a type switch. Consider the following example:

func aa() (*int, error) {
	return new(int), nil
}

func bb() {
	ptr, err := aa()
	if err == nil {
		print(*ptr) // OK, no NilAway errors.
	}
	switch err.(type) {
	case nil:
		print(*ptr) // Also OK, but NilAway does report an error.
                // analysistest.go:459: go.uber.org/errorreturn/inference/errorreturn-with-inference.go:191:10: unexpected diagnostic:  Value returned as result 0 from the function `aa` lacking guarding (definitely nilable) and is dereferenced at "inference/errorreturn-with-inference.go:191:10" (must be nonnil)
	}
}

The case nil in the type switch has the same effect as a normal if statement check if err == nil, but NilAway doesn't seem to understand this fact.