ada: Allow reuse of Enclosing_Declaration_Or_Statement by GNATprove

ada: Allow reuse of Enclosing_Declaration_Or_Statement by GNATprove

Move routine Enclosing_Declaration_Or_Statement from body of Sem_Res to spec
of Sem_Util, so it can be reused. In particular, GNATprove needs this
functionality to climb from an arbitrary subexpression with target_name (@)
to the enclosing assignment statement. Behaviour of the compiler is
unaffected.

gcc/ada/

	* sem_res.adb (Enclosing_Declaration_Or_Statement): Moved to
	Sem_Util.
	* sem_util.ads (Enclosing_Declaration_Or_Statement): Moved from
	Sem_Res.
	* sem_util.adb (Enclosing_Declaration_Or_Statement): Likewise.