[Ada] Fix some handling of warnings

[Arnaud Charlet <charlet at adacore dot com>]

Tested on x86-linux, committed on mainline.

The fact that a variable is aliased or renamed causes the value of the
variable to be available through several entities. Thus, in the case of
aliased or renamed values, no warning is emitted. However, the fact that
a variable has an access type, or a private type whose full view is an
access type, does not in itself introduce potential multiple access paths
to its value, and therefore must not cause suppression of this warning.

Test case zq.adb: gcc -c -gnatwa zq.adb must accept source with no
error and the following three warnings:
zq.adb:4:04: warning: variable "X1" is assigned but never read
zq.adb:5:04: warning: variable "X2" is assigned but never read
zq.adb:6:04: warning: variable "X3" is assigned but never read
--
package ZP is
   type T1 is private; type T2 is private; type T3 is access Integer;
private
   type T1 is access Integer; type T2 is null record;
end ZP;
with ZP;
procedure ZQ is
   X1, Y1 : ZP.T1; X2, Y2 : ZP.T2; X3, Y3 : ZP.T3;
begin
   Y3 := null; --  Kill no-assignment warning
   X1 := Y1; X2 := Y2; X3 := Y3;
end ZQ;

2004-12-07  Thomas Quinot  <quinot@adacore.com>
	    Ed Schonberg  <schonberg@adacore.com>

	* exp_ch4.adb (Expand_N_Indexed_Component): For an indexed component
	with an implicit dereference as its prefix, use
	Insert_Explicit_Dereference instead of merely rewriting the prefix into
	an explicit dereference. This ensures that a reference to the original
	prefix is generated, if appropriate.

	* sem_util.adb (Insert_Explicit_Dereference): When an implicit
	dereference is rewritten to an explicit one, generate a reference to
	the entity denoted by its prefix using the original prefix node, so
	the dereference can be properly recorded as a read of the denoted
	access value, if appropriate.

	* sem_warn.adb (Output_Unreferenced_Messages): Do not abstain from
	emitting 'assigned but never read' warning on a variable on the basis
	that it has an access type.
	(Check_References): Emit unreferenced warning when the scope is a
	subprogram body.

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