[Ada] Fix for atomic wrongly rejected on object of discriminated type

[Pierre-Marie de Rodat]

The reason is that the automatic alignment promotion is not yet performed
in the case where the nominal subtype is of variable size.

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

gcc/ada/

	* gcc-interface/decl.c (promote_object_alignment): Add GNU_SIZE
	parameter and use it for the size of the object if not null.
	(gnat_to_gnu_entity) <E_Variable>: Perform the automatic alignment
	promotion for objects whose nominal subtype is of variable size.
	(gnat_to_gnu_field): Adjust call to promote_object_alignment.
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