compiler: type of string index expression is byte

compiler: type of string index expression is byte

To make this work from the do_type method, add "byte" and "rune" to
the list of known integer types, and look them up that way rather than
via gogo->lookup_global.

For golang/go#8745

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/275653