Fix internal error on small array with negative lower bound

Ada supports arrays with negative indices, although the internal index type
is sizetype like in other languages, which is unsigned.  This means that
negative values are represented by very large numbers, which works with a
bit of care.  This plugs a small loophole in output_constructor_bitfield.

gcc/
	* varasm.cc (output_constructor_bitfield): Call tree_to_uhwi instead
	of tree_to_shwi on array indices.  Minor tweaks.

gcc/testsuite/
	* gnat.dg/specs/array6.ads: New test.

gcc/testsuite/gnat.dg/specs/array6.ads

@@ -0,0 +1,14 @@
+-- { dg-do compile }
+
+package Array6 is 
+
+  type Range_Type is range -10 ..  10;
+  type Array_Type is array (Range_Type range <> ) of Short_Short_Integer;
+
+  type Record_Type is record 
+    A : Array_Type(-2..4);
+  end record ;
+
+  Rec : Record_Type := (A => (others => -1));
+
+end Array6;

gcc/varasm.cc

@@ -5564,19 +5564,18 @@ output_constructor_bitfield (oc_local_state *local, unsigned int bit_offset)
 
   /* Relative index of this element if this is an array component.  */
   HOST_WIDE_INT relative_index
-    = (!local->field
-       ? (local->index
-	  ? (tree_to_shwi (local->index)
-	     - tree_to_shwi (local->min_index))
-	  : local->last_relative_index + 1)
-       : 0);
+    = (local->field
+       ? 0
+       : (local->index
+	  ? tree_to_uhwi (local->index) - tree_to_uhwi (local->min_index)
+	  : local->last_relative_index + 1));
 
   /* Bit position of this element from the start of the containing
      constructor.  */
   HOST_WIDE_INT constructor_relative_ebitpos
-      = (local->field
-	 ? int_bit_position (local->field)
-	 : ebitsize * relative_index);
+    = (local->field
+       ? int_bit_position (local->field)
+       : ebitsize * relative_index);
 
   /* Bit position of this element from the start of a possibly ongoing
      outer byte buffer.  */