[Ada] Lift restriction on renaming with Volatile_Full_Access

[Eric Botcazou]

> An arbitrary restriction was imposed on renaming in conjunction with the new
> Aspect/Pragma Volatile_Full_Access for implementation reasons: the compiler
> was rejecting renamings of components of Volatile_Full_Access objects.

The reason was that supporting Volatile_Full_Access requires an overhaul of 
the implementation of renaming is gigi because the current approach falls back 
to references in some cases and these are not compatible with VFA.

This is the first part of this overhaul, tested on x86_64-suse-linux, applied 
on the mainline.


2015-05-26  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/gigi.h (gnat_stabilize_reference): Adjust prototype.
	* gcc-interface/decl.c (gnat_to_gnu_entity): Do not rely on const_flag
 	to detect constant renamings.  Be prepared for specific pattern of
	renamed object based on function calls.  Create a constant object
	for the renaming of a NULL_EXPR or of a CONSTRUCTOR.  Adjust calls
	to gnat_stabilize_reference and tidy up.  Remove redundant tests.
	(elaborate_expression_1): Remove obsolete test and tidy up.
	* gcc-interface/trans.c (Call_to_gnu): Do not stabilize In/Out or Out
	parameters passed by reference.
	(gnat_to_gnu) <N_Selected_Component>: Remove redundant protection again
	side-effects.
	Use gnat_protect_expr instead of gnat_stabilize_reference for general
	protection against side-effects.
	* gcc-interface/utils2.c (gnat_stable_expr_p): New predicate.
	(gnat_save_expr): Invoke it.
	(gnat_protect_expr): Likewise.
	(gnat_stabilize_reference_1): Likewise.  Remove useless propagation
	of TREE_THIS_NOTRAP.
	(gnat_stabilize_reference): Remove parameter and adjust throughout.
	Delete ADDR_EXDR, COMPOUND_EXPR and CONSTRUCTOR cases.
	Restrict CALL_EXPR case to atomic loads and tweak ERROR_MARK case.


-- 
Eric Botcazou
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