Leverage type information provided by type assertions

[mlevesquedion]

This issue is related to #161.

Consider the following test case:

func TestPanicTypeAssertSource(i interface{}) {
	s := i.(core.Source)
	_ = s
	// The dominating type assertion would panic if i were not a source type.
	core.Sink(i) // TODO(210) want "a source has reached a sink"
}

Clearly, the only way that the call to core.Sink can be reached is if i holds a value of type core.Source. The above code is therefore unsafe.

The following case is more ambiguous, and currently we do not expect a report to be produced:

func TestOkayTypeAssert(i interface{}) {
	s, ok := i.(core.Source)
	_, _ = s, ok
	// The dominating type assertion will not panic.
	core.Sink(i)
}

Since the , ok form is used, it is possible that the type assertion failed and that i is in fact safe to sink. However, there is certainly a possibility that i contains a value of type core.Source, and therefore this code should perhaps be considered unsafe.