[Ada] Crash on implicit dereference not made explicit

[Ada] Crash on implicit dereference not made explicit

2019-12-13  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve): IF an entity reference is overloaded
	because its type has an Implicit_Dereference aspect, we must
	examine the discriminants of the type to determine whether an
	explicit dereference must be inserted for use in code
	generation. Previously this was done for other expressions but
	not for entity references by themselves.  This was sufficient to
	handle uses of the aspect in container handling and iteration,
	but not more generally.

From-SVN: r279352