[Ada] Do not generate dangling references to bounds

[Eric Botcazou]

This prevents gigi from generating dangling references to the bounds of an 
aliased parameter of an unconstrained array type.  This cannot happen in 
strict Ada but you can bypass the rules by means of 'Unchecked_Access.

Tested on x86_64-suse-linux, applied on the mainline and 9 branch.


2019-05-27  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/trans.c (Identifier_to_gnu): Minor tweaks.
	(gnat_to_gnu): Do not convert the result if it is a reference to an
	unconstrained array used as the prefix of an attribute reference that
	requires an lvalue.


2019-05-27  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/aliased2.adb: New test.

-- 
Eric Botcazou
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