compiler: add support for anonymous type asserts

This is used for example by the errors package, which contains:

    if x, ok := err.(interface{ As(interface{}) bool }); ok && x.As(target) {
        return true
    }

The interface here is not a named type.

testdata/interface.txt

@@ -18,6 +18,7 @@ is Tuple: 0 8 16 24
 SmallPair.Print: 3 5
 Stringer.String(): foo
 Stringer.(*Thing).String(): foo
+s has String() method: foo
 nested switch: true
 non-blocking call on sometimes-blocking interface
 slept 1ms