[Ada] Ineffective use type clause warnings cause compile time crash

[Ada] Ineffective use type clause warnings cause compile time crash

This patch corrects an error in the compiler whereby the presence of a
generic instance featuring a use type clause at library level may cause
a crash at compile time when warnings for ineffective use clauses are
enabled and the type in question is already use visible.

gcc/ada/

	* sem_ch8.adb (Determine_Package_Scope): Created to centralize
	the calculation of which package a given use clause belongs to.
	(Most_Descendant_Use_Clause): Modified to call
	Determine_Package_Scope.
	* sem_util.adb, sem_util.ads (Enclosing_Package): Modified to
	handle both entity and node ids.