[Ada] Fix analysis of access objects in Depends contracts

[Ada] Fix analysis of access objects in Depends contracts

gcc/ada/

	* sem_prag.adb (Find_Role): Constant object of
	access-to-constant and access-to-subprogram types are not
	writable.
	(Collect_Subprogram_Inputs_Outputs): In-parameters of
	access-to-variable type can act as outputs of the Depends
	contracts.