ada: Reject nonconfirming Size attribute value for aliased object

Only confirming Size must be supported for aliased object of elementary
type (see RM 13.1 in the "Implementation Advice").

   -- size is 1-byte
   type Y is range 0 .. 20;
   type Ay is access all Y;

   -- Var size is 8-bytes
   Var : aliased Y := 5 with Size => 64;

   --  JP.all is a 1-byte reference to an 8-bytes objects.
   JP : Ay := Var'Access;

The above JP.all references the first byte of the 8-byte Var object,
which is, for example, not correct on little-endian systems.

This change rejects nonconfirming Size attribute on such objects
instead of miscompiling it.

gcc/ada/

	* sem_ch13.adb (Check_One_Attr): produce error when Size attribute
	used on aliased object of elementary types with nonconfirming
	value.