Fix uniqueness of address for aliased objects

Fix uniqueness of address for aliased objects

Two aliased objects must have distinct addresses, even if they have
size zero, so we make sure to allocate at least one byte for them.

	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Variable>: Force at
	least the unit size for an aliased object of a constrained nominal
	subtype whose size is variable.