Doxygenize the documentation comments

src/IceRegAlloc.cpp

 //===- subzero/src/IceRegAlloc.cpp - Linear-scan implementation -----------===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
-//
-// This file implements the LinearScan class, which performs the
-// linear-scan register allocation after liveness analysis has been
-// performed.
-//
+///
+/// \file
+/// This file implements the LinearScan class, which performs the
+/// linear-scan register allocation after liveness analysis has been
+/// performed.
+///
 //===----------------------------------------------------------------------===//
 
 #include "IceRegAlloc.h"
 
 #include "IceCfg.h"
 #include "IceCfgNode.h"
 #include "IceInst.h"
 #include "IceOperand.h"
 #include "IceTargetLowering.h"
 
@@ skipping 735 matching lines @@
     Str << "\n";
   }
   Str << "++++++ Inactive:\n";
   for (const Variable *Item : Inactive) {
     dumpLiveRange(Item, Func);
     Str << "\n";
   }
 }
 
 } // end of namespace Ice