GNAT does not issue a warning anymore on a postcondition of True (used
here to prevent inining inside GNATprove for proof).

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

gcc/ada/

	* libgnat/s-valuei.ads: Remove pragma Warnings Off.
	* libgnat/s-valueu.ads: Same.
	* libgnat/s-valuti.ads: Same.