[Ada] Improvement to errors and warning messages for expanded names

[Arnaud Charlet <charlet at adacore dot com>]

This change modifies the way expanded_names are reported in error messages:
the full expanded name, in source form, is now quoted, rather than just
its terminal Selector_Name.

The following compilation must produce the shown warning message, including
the full name 'Ada.Calendar':

$ gcc -c trygnatwu.adb -gnatwu
trygnatwu.adb:2:09: warning: no entities of "Ada.Calendar" are referenced

with Ada.Text_Io; use Ada.Text_Io;
with Ada.Calendar; use Ada.Calendar;
procedure trygnatwu is
begin
   Put_Line("Hello");
end trygnatwu;

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

2012-01-23  Thomas Quinot  <quinot@adacore.com>

	* errout.adb (Set_Msg_Node): For an N_Expanded_Name, output
	the complete expanded name, rather than just its Selector_Name.

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