ada: Fix fallout of recent fix for missing finalization

ada: Fix fallout of recent fix for missing finalization

The original fix makes it possible to create transient scopes around return
statements in more cases, but it overlooks that transient scopes are reused
and, in particular, that they can be promoted to secondary stack management.

gcc/ada/

	* exp_ch7.adb (Find_Enclosing_Transient_Scope): Return the index in
	the scope table instead of the scope's entity.
	(Establish_Transient_Scope): If an enclosing scope already exists,
	do not set the Uses_Sec_Stack flag on it if the node to be wrapped
	is a return statement which requires secondary stack management.