Doxygenize the documentation comments

src/IceTargetLoweringX8632.cpp

 //===- subzero/src/IceTargetLoweringX8632.cpp - x86-32 lowering -----------===//
 //
 //                        The Subzero Code Generator
 //
 //===----------------------------------------------------------------------===//
-//
-// This file implements the TargetLoweringX8632 class, which
-// consists almost entirely of the lowering sequence for each
-// high-level instruction.
-//
+///
+/// \file
+/// This file implements the TargetLoweringX8632 class, which
+/// consists almost entirely of the lowering sequence for each
+/// high-level instruction.
+///
 //===----------------------------------------------------------------------===//
 
 #include "IceTargetLoweringX8632.h"
 
 #include "IceTargetLoweringX86Base.h"
 
 namespace Ice {
 namespace X86Internal {
 template <> struct MachineTraits<TargetX8632> {
   using InstructionSet = TargetX8632::X86InstructionSet;
@@ skipping 402 matching lines @@
 // entries in case the high-level table has extra entries.
 #define X(tag, size, align, elts, elty, str)                                   \
   static_assert(_table1_##tag == _table2_##tag,                                \
                 "Inconsistency between ICETYPEX8632_TABLE and ICETYPE_TABLE");
 ICETYPE_TABLE
 #undef X
 } // end of namespace dummy3
 } // end of anonymous namespace
 
 } // end of namespace Ice