[Ada] Attribute 'Old should only be used in postconditions

[Arnaud Charlet <charlet at adacore dot com>]

Forbid the use of attribute 'Old outside of postconditions, as required in
Ada 2012. Similarly as for 'Result, it may also be used in Ensures clauses
of test cases and contract cases attached to the subprogram. There is no need
anymore to detect specially references to local variables, which would be
rejected anyway in the context in which preconditions and postconditions are
analyzed.

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

2012-06-26  Yannick Moy  <moy@adacore.com>

	* sem_attr.adb (Analyze_Attribute): Detect if 'Old is used outside a
	postcondition, and issue an error in such a case.

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