[Ada] Diagnose unsupported bit packed arrays

[Arnaud Charlet <charlet at adacore dot com>]

This change adds circuitry to identify some bit packed arrays that are
known at compile time to exceed the maximum size supported by the
implementation.

The following compilation must be rejected with the indicated error message:
$ gcc large_bit_packed_array.ads
large_bit_packed_array.ads:2:09: bit packed array type may not have more than
  Integer'Last+1 elements

package Large_Bit_Packed_Array is
   type A is array (Integer) of Boolean;
   pragma Pack (A);
end Large_Bit_Packed_Array;

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

2013-04-23  Thomas Quinot  <quinot@adacore.com>

	* freeze.adb (Freeze_Entity): For the case of a bit-packed
	array time that is known at compile time to have more that
	Integer'Last+1 elements, issue an error, since such arrays are
	not supported.

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