ada: Cope with Sem_Util.Enclosing_Declaration oddness.

ada: Cope with Sem_Util.Enclosing_Declaration oddness.

Sem_Util.Enclosing_Declaration can return a non-empty result which is not
a declaration; clients may need to compensate for the case where an
N_Subprogram_Specification node is returned. One such client is the function
Is_Actual_Subp_Of_Inst.

gcc/ada/

	* sem_ch8.adb (Is_Actual_Subp_Of_Inst): After calling
	Enclosing_Declaration, add a check for the case where one more
	Parent call is needed to get the enclosing declaration.