[Ada] Crash on illegal current instance

[Pierre-Marie de Rodat <derodat at adacore dot com>]

If the type_mark of a qualified_expression refers to the current
instance of the type, do not crash; instead give a proper error
message. This is illegal by RM-8.6(17).

The following test should get an error:

current_instance_default.ads:2:54: current instance not allowed

package Current_Instance_Default is
   type Color is (Red, Orange) with Default_Value => Color'(Red); -- ERROR:
end Current_Instance_Default;

Tested on x86_64-pc-linux-gnu, committed on trunk

2017-09-18  Bob Duff  <duff@adacore.com>

	* sem_ch4.adb (Analyze_Qualified_Expression): Give an error if the type
	mark refers to the current instance. Set the type to Any_Type in that
	case, to avoid later crashes.

Attachment: difs
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