go/types: complex(0, 0i) should be permitted

gc successfully compiles the following program:

``` go
package a
var e = complex(0, 0i)
```

while gotype rejects it with:

```
invalid argument: 0i (untyped complex constant 0) must be a float32, float64, or an untyped non-complex numeric constant
```

go version devel +9b04852 Sat Jul 11 00:08:50 2015 +0000 linux/amd64
