[Ada] Fix a bug in the contract of formal ordered sets

[Ada] Fix a bug in the contract of formal ordered sets

There are 2 main issues in the postcondition of the function Replace
of the formal ordered sets, in the sub package Generic_Keys. One is
related to the fact that when the element is changed, the key is
also changed. The second one is due to the fact that the arrival of
the new element might modify the ordering of the set and thus
the Positions might be modified.
As a consequence, the postcondition might be false and thus fail
at runtime.

gcc/ada/

	* libgnat/a-cforse.ads (Replace): Fix the postcondition.