Support validation via custom bool functions

[sonalmahajan15]

Currently, NilAway does not track nilability checked via custom boolean returning functions. Hence, this results in false positives, such as in the example shown below.

func isValNil(val *int) bool {
	return val == nil
}

func test() {
	var x *int
	if !isValNil(x) {
		_ = *x // FP: unassigned variable `x` dereferenced
	}
}

[sonalmahajan15]

Duplicate of issue #212 .