[Ada] fix crash on Secondary_Stack_Size with discriminant

[Ada] fix crash on Secondary_Stack_Size with discriminant

This patch fixes a crash caused by specifying the Secondary_Stack_Size
aspect of a task type as the value of a discriminant of the task type,
and then declaring a record component whose type is the task type,
constrained to a discriminant of the record.

gcc/ada/

	* sem_ch3.adb (Get_Discr_Value): Copy the result. Otherwise, the
	"tree" can be malformed (a DAG, not a tree) because
	Get_Discr_Value could be returning some subtree already attached
	to the tree.