[Ada] Constraints on class-wide types are ignored

[Arnaud Charlet <charlet at adacore dot com>]

A class-wide type has anonymous discriminants, because type extensions can
add discriminants at will.  A constraint on a class-wide type is thus a partial
constraint that applies only to the known discriminants of the root type. Such
a partial constraint is a language pathology that the ARG has decided not to
test. This patch simply discards such a constraint on an access type, so that
the designated type includes all (unconstrained) extensions of the root type.

The following must compile with the warning:

   volumes.ads:9:24: warning: constraint on class-wide type ignored

---
package Volumes is
   type VolumeWidgetType (Stereo : boolean) is tagged
      record
        IsStereo : boolean := Stereo;
      end record;

   type VolumeWidget is access all VolumeWidgetType'Class;

   Mic1 : VolumeWidget (Stereo => False);

end Volumes;

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

2013-09-10  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch3.adb (Process_Subtype): Discard constraint on access
	to class-wide type. Such constraints are not supported and are
	considered a language pathology.

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