[Ada] Improve warning on address clauses

[Arnaud Charlet <charlet at adacore dot com>]

Tested on x86-linux, committed on mainline.

Previously, warnings were generated only if an object address violated an
explicit alignment constraint put on the object itself. In some cases, this
new check allows the compiler not to generate a run time check as it knows
that the object is correctly aligned.

test case:
$ gcc -c test_alignment.ads
test_alignment.ads:6:65: warning: specified address for "Y" not consistent with alignment (RM 13.3(27))
--
with System.Storage_Elements;
package Test_Alignment is
   type T is new Integer;
   for T'Alignment use 8;
   X : T; for X'Address use System.Storage_Elements.To_Address (32);
   Y : T; for Y'Address use System.Storage_Elements.To_Address (33);
end Test_Alignment;

2005-01-03  Samuel Tardieu  <tardieu@adacore.com>

	* checks.adb (Apply_Alignment_Check): Generate a warning if an object
	address is incompatible with its base type alignment constraints when
	this can be decided statically.

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