[Ada] Ignore pragmas when looking for unreachable code

[Arnaud Charlet <charlet at adacore dot com>]

This patch causes pragmas to be ignored when checking for unreachable
code, which catches additional cases not caught before, as shown by
the following example, compiled with -gnatwe.w -gnatld7

     1. with Ada.Text_IO; use Ada.Text_IO;
     2. procedure Main4 is
     3. begin
     4.    loop
     5.       Put_Line ("Main");
     6.    end loop;
     7.
     8.    pragma List (On);
     9.    Put_Line ("end");
           |
        >>> warning: unreachable code

    10. end Main4;

This also fixes an anomoly that trying to suppress the warning with
pragma Warnings (Off, text) did not work, since the pragma itself
killed the warning.

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

2012-04-26  Robert Dewar  <dewar@adacore.com>

	* sem_ch5.adb (Check_Unreachable_Code): Skip past pragmas.

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