Builtin_call_expression::do_numeric_constant_value can be called by
    Array_type::verify_length before the determine types pass, so accept
    an abstract type.
    
    Test case is https://golang.org/cl/147537.
    
    Fixes golang/go#28601
    
    Reviewed-on: https://go-review.googlesource.com/c/147442

From-SVN: r265820