compiler: handle abstract type in builtin numeric const value

    
    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

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-df841cce9fd271a25269e3514c2e9b61deaf2c4c
+5fcfe352ad91945a4f4d0dcfb6309df9bd072c7d
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -8294,7 +8294,7 @@ Builtin_call_expression::do_numeric_constant_value(Numeric_constant* nc) const
       if (arg_type->is_error())
 	return false;
       if (arg_type->is_abstract())
-	return false;
+	arg_type = arg_type->make_non_abstract_type();
       if (this->seen_)
         return false;