[Ada] Improve generated code for records with rep clauses

[Eric Botcazou]

This makes sure the handling of bit-fields is uniform in packed records or 
records with representation clause.

Tested on x86-64/Linux, applied on mainline,


2018-06-12  Eric Botcazou  <ebotcazou@adacore.com>

	* gcc-interface/decl.c (variant_desc): Add AUX field.
	(gnat_to_gnu_entity) <discrete_type>: Do not call compute_record_mode
	directly.
	(reverse_sort_field_list): New static function.
	(components_to_record): Place the variant part at the beginning of the
	field list when there is an obvious order of increasing position.
	(build_variant_list): Initialize it.
	(create_variant_part_from): Do not call compute_record_mode directly.
	(copy_and_substitute_in_layout): Likewise.  Always sort the fields with
	fixed position in order of increasing position, in the record and all
	the variants, in any.  Call reverse_sort_field_list.
	* gcc-interface/utils.c (make_packable_type): Compute the sizes before
	calling finish_record_type.  Do not call compute_record_mode directly.
	(finish_record_type): Overhaul final processing depending on REP_LEVEL
	and call finish_bitfield_layout if it is equal to one or two.


2018-06-12  Eric Botcazou  <ebotcazou@adacore.com>

	* gnat.dg/opt72a.ad[sb]: New test.
	* gnat.dg/opt72_pkg.ads: New helper.

-- 
Eric Botcazou
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