Due to a latent bug with freezing, the expression function with
Subprogram_Variant which was recently added to the System.Val_Util unit
triggers a crash in CodePeer.

Ordinary compilation was not affected by this bug, because of the
Assertion_Policy (Ghost => Ignore) applied to this unit.

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

gcc/ada/

	* libgnat/s-valuti.ads (Scan_Natural_Ghost): Split body from
	spec and put it into private part, so that GNATprove can pick it
	both when analysing the unit and its clients.