[Ada] Implicit dereferencing in container indexing

[Ada] Implicit dereferencing in container indexing

gcc/ada/

	* sem_ch4.adb (Try_Container_Indexing): When the prefix type is
	an access type, change it to the designated type, change the
	prefix to an explicit dereference, and emit a ?d? warning for
	the implicit dereference. Include a ??? comment questioning
	whether this is the right context in which to perform the
	implicit dereferencing.