[Ada] Derived protected types with discriminants

[Arnaud Charlet <charlet at adacore dot com>]

This patch handles derived protected types that have fewer discriminants than
the parent type. In this case the corresponding record of the derived type
must be a similar derivation of the corresponding record of the parent

The following must compile quietly:

package Der_Prot  is
   protected type Object( A, B: Positive )
   is
      entry Wait;
      procedure Send;
   private
     Name1 : String (1 .. A) := (others => '+');
     Name2 : String (1 .. B) := (others => '?');
   end Object;

   type Object2 (A, B : Positive) is new Object (B, A);

   type Object3( A : Positive ) is new Object ( A => A, B => 2);
end Der_Prot ;

package body Der_Prot is
   protected body Object
   is
      entry Wait when true is
      begin
         null;
      end Wait;

      procedure Send is
      begin
         null;
      end Send;
   end Object;
end Der_Prot;

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

2009-07-28  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch3.adb (Build_Derived_Concurrent_Type): Handle properly a
	derivation that renames some discriminants and constrain others.
	* exp_ch9.adb (Build_Protected_Subprogram_Call): If the type of the
	prefix is a derived untagged type, convert to the root type to conform
	to the signature of the protected operations.

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