[Ada] Improve efficiency of aggregates with <>

The generated code has been improved so that aggregates with <> are more
efficient.  No change in behavior; no test.

2019-08-20  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_aggr.adb (Expand_Array_Aggregate): Use build-in-place in
	the nonlimited case in STEP 4. This improves the efficiency of
	things like (1 .. 1000 => <>).  We still generate some code for
	that, unfortunately, but it is much improved.
	(Aggr_Assignment_OK_For_Backend): Return false if <> components
	are present.

From-SVN: r274739

gcc/ada/ChangeLog

 2019-08-20  Bob Duff  <duff@adacore.com>
 
+	* exp_aggr.adb (Expand_Array_Aggregate): Use build-in-place in
+	the nonlimited case in STEP 4. This improves the efficiency of
+	things like (1 .. 1000 => <>).  We still generate some code for
+	that, unfortunately, but it is much improved.
+	(Aggr_Assignment_OK_For_Backend): Return false if <> components
+	are present.
+
+2019-08-20  Bob Duff  <duff@adacore.com>
+
 	* exp_ch6.adb (Needs_BIP_Alloc_Form): Call
 	Requires_Transient_Scope rather than checking constrainedness
 	and so forth.  We have previously improved

gcc/ada/exp_aggr.adb

@@ -5233,6 +5233,12 @@ package body Exp_Aggr is
          Value     : Uint;
 
       begin
+         --  Back end doesn't know about <>
+
+         if Has_Default_Init_Comps (N) then
+            return False;
+         end if;
+
          --  Recurse as far as possible to find the innermost component type
 
          Ctyp := Etype (N);
@@ -6292,9 +6298,7 @@ package body Exp_Aggr is
       --  previously excluded controlled components but this is an old
       --  oversight: the rules in 7.6 (17) are clear.
 
-      if (not Has_Default_Init_Comps (N)
-           or else Is_Limited_Type (Etype (N)))
-        and then Comes_From_Source (Parent_Node)
+      if Comes_From_Source (Parent_Node)
         and then Parent_Kind = N_Object_Declaration
         and then Present (Expression (Parent_Node))
         and then not