ada: Fix memory leak in expression function returning Big_Integer

ada: Fix memory leak in expression function returning Big_Integer

We fail to establish a transient scope around the return statement because
the function returns a controlled type, but this is no longer problematic
because controlled types are no longer returned on the secondary stack.

gcc/ada/

	* exp_ch7.adb (Establish_Transient_Scope.Find_Transient_Context):
	Bail out for a simple return statement only if the transient scope
	and the function both require secondary stack management, or else
	if the function is a thunk.
	* sem_res.adb (Resolve_Call): Do not create a transient scope when
	the call is the expression of a simple return statement.