compiler: emit underlying constant in array_type length export

    
    In Array_type::do_export, when emitting a concrete array length,
    evaluate the length expression to an integer constant and emit that
    constant, instead of calling the more general method for emitting
    expressions. This is to avoid the possibility that we will need
    to emit a conversion, which could confuse the gccgoimporter.
    
    Fixes golang/go#30628.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/165741

From-SVN: r269443

gcc/go/gofrontend/MERGE

-13c98c3477647888fc7a186e9055793b0961e806
+959260238817af3205fb9907dd92319291e6a893
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/types.cc

@@ -7581,10 +7581,17 @@ Array_type::do_export(Export* exp) const
   exp->write_c_string("[");
   if (this->length_ != NULL)
     {
-      Export_function_body efb(exp, 0);
-      efb.set_type_context(this->length_->type());
-      this->length_->export_expression(&efb);
-      exp->write_string(efb.body());
+      Numeric_constant nc;
+      mpz_t val;
+      if (!this->length_->numeric_constant_value(&nc) || !nc.to_int(&val))
+        {
+	  go_assert(saw_errors());
+          return;
+        }
+      char* s = mpz_get_str(NULL, 10, val);
+      exp->write_string(s);
+      exp->write_string(" ");
+      mpz_clear(val);
     }
   exp->write_c_string("] ");
   exp->write_type(this->element_type_);