[Ada] Fix compiler crash on function returning array at -O

[Eric Botcazou]

This is a regression present on all active branches.  The compiler segfaults 
on a function returning an array variable declared locally and used in a 
grandchild function but not in its parent function (which is a child of the 
first function).

Tested on x86_64-suse-linux, applied on all active branches.


2019-05-28  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/trans.c (walk_nesting_tree): New static function.
	(finalize_nrv): Use it to walk the entire nesting tree.


2019-05-28  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/opt79.ad[sb]: New test.

-- 
Eric Botcazou
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