No longer report a problem if target's type is interface{}.
This avoids false positives like
```
var e error
var i interface{} = &e
... errors.As(..., i) ...
```
Change-Id: Ibf6e7163147248305130a5e650f92b80e34a44de
Reviewed-on: https://go-review.googlesource.com/c/tools/+/175717
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
b1dcc6b189