[Ada] Avoid gnatbind regression caused by Copy_Bitfield

[Pierre-Marie de Rodat <derodat@adacore.com>]

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The recent Copy_Bitfield change caused gnatbind to change elaboration
order, causing different error messages.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-09-18  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_ch5.adb (Expand_Assign_Array_Loop_Or_Bitfield): Move call
	to RTE_Available later, so it doesn't disturb the elab order.
	The RE_Copy_Bitfield entity is defined in package
	System.Bitfields which has a dependency on package
	System.Bitfield_Utils, which has it its spec:

	   pragma Elaborate_Body;

	The query on RTE_Available forces loading and analyzing
	System.Bitfields and all its withed units.

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--- gcc/ada/exp_ch5.adb
+++ gcc/ada/exp_ch5.adb
@@ -1475,8 +1475,7 @@ package body Exp_Ch5 is
       --  optimization in that case as well.  We could complicate this code by
       --  actually looking for such volatile and independent components.
 
-      if RTE_Available (RE_Copy_Bitfield)
-        and then Is_Bit_Packed_Array (L_Type)
+      if Is_Bit_Packed_Array (L_Type)
         and then Is_Bit_Packed_Array (R_Type)
         and then not Reverse_Storage_Order (L_Type)
         and then not Reverse_Storage_Order (R_Type)
@@ -1489,6 +1488,7 @@ package body Exp_Ch5 is
         and then not Has_Independent_Components (R_Type)
         and then not L_Prefix_Comp
         and then not R_Prefix_Comp
+        and then RTE_Available (RE_Copy_Bitfield)
       then
          return Expand_Assign_Array_Bitfield
            (N, Larray, Rarray, L_Type, R_Type, Rev);