ada: Fix bogus Storage_Error on dynamic array with static zero length

ada: Fix bogus Storage_Error on dynamic array with static zero length

This works around the limitations present for the support of arrays in the
middle-end by clearing the TREE_OVERFLOW flag for arrays with zero length.

gcc/ada/

	* gcc-interface/decl.cc (gnat_to_gnu_entity) <E_Array_Type>: Use a
	local variable for the GNAT index type.
	<E_Array_Subtype>: Likewise.  Call Is_Null_Range on the bounds and
	force the zero on TYPE_SIZE and TYPE_SIZE_UNIT if it returns true.