[Ada] Fix to SCOs for SELECT alternatives with guard

This change fixes incorrect source location indications in SCOs for decisions
corresponding to guard expressions in SELECT statements, which can cause
a crash in coverage analysis tools.

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

2012-07-23  Thomas Quinot  <quinot <at> adacore.com>

	* par_sco.adb (Process_Decisions.Output_Header): For the guard
	on an alternative in a SELECT statement, use the First_Sloc
	of the guard expression (not its topmost sloc) as the decision
	location, because this is what is referenced by dominance markers.

Index: par_sco.adb
===================================================================
--- par_sco.adb	(revision 189768)
+++ par_sco.adb	(working copy)
 <at>  <at>  -25,6 +25,7  <at>  <at> 

 with Atree;    use Atree;
 with Debug;    use Debug;
+with Errout;   use Errout;
 with Lib;      use Lib;
 with Lib.Util; use Lib.Util;
 with Namet;    use Namet;
 <at>  <at>  -495,13 +496,15  <at>  <at> 
                --  levels (through the pragma argument association) to get to
                --  the pragma node itself. For the guard on a select