errorsas: ignore empty interface target

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>

go/analysis/passes/errorsas/errorsas.go

@@ -56,9 +56,13 @@ func run(pass *analysis.Pass) (interface{}, error) {
 
 var errorType = types.Universe.Lookup("error").Type().Underlying().(*types.Interface)
 
-// pointerToInterfaceOrError reports whether the type of e is a pointer to an interface or a type implementing error.
+// pointerToInterfaceOrError reports whether the type of e is a pointer to an interface or a type implementing error,
+// or is the empty interface.
 func pointerToInterfaceOrError(pass *analysis.Pass, e ast.Expr) bool {
 	t := pass.TypesInfo.Types[e].Type
+	if it, ok := t.Underlying().(*types.Interface); ok && it.NumMethods() == 0 {
+		return true
+	}
 	pt, ok := t.Underlying().(*types.Pointer)
 	if !ok {
 		return false

go/analysis/passes/errorsas/testdata/src/a/a.go

@@ -24,11 +24,13 @@ func _() {
 		m  myError
 		i  int
 		f  iface
+		ei interface{}
 	)
-	errors.As(nil, &e)
-	errors.As(nil, &m)
-	errors.As(nil, &f)
-	errors.As(nil, perr())
+	errors.As(nil, &e)     // *error
+	errors.As(nil, &m)     // *T where T implemements error
+	errors.As(nil, &f)     // *interface
+	errors.As(nil, perr()) // *error, via a call
+	errors.As(nil, ei)     //  empty interface
 
 	errors.As(nil, nil) // want `second argument to errors.As must be a pointer to an interface or a type implementing error`
 	errors.As(nil, e)   // want `second argument to errors.As must be a pointer to an interface or a type implementing error`