Doxygenize the documentation comments

src/IceFixups.cpp

 //===- subzero/src/IceFixups.cpp - Implementation of Assembler Fixups -----===//
 //
 //                        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 AssemblerFixup class, a very basic
-// target-independent representation of a fixup or relocation.
-//
+///
+/// \file
+/// This file implements the AssemblerFixup class, a very basic
+/// target-independent representation of a fixup or relocation.
+///
 //===----------------------------------------------------------------------===//
 
 #include "IceFixups.h"
 
 #include "IceOperand.h"
 
 namespace Ice {
 
 const Constant *AssemblerFixup::NullSymbol = nullptr;
 
@@ skipping 31 matching lines @@
   if (isNullSymbol())
     Str << "__Sz_AbsoluteZero";
   else
     Str << symbol(Ctx);
   RelocOffsetT Offset = offset() + BaseOffset;
   if (Offset)
     Str << " + " << Offset;
 }
 
 } // end of namespace Ice