[Ada] Improve efficiency of aggregates with <>

[Pierre-Marie de Rodat]

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

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

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.
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