fix doxygen brief in subzero header files

src/IceTargetLoweringMIPS32.h

 //===- subzero/src/IceTargetLoweringMIPS32.h - MIPS32 lowering ---*- C++-*-===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
-/// This file declares the TargetLoweringMIPS32 class, which implements the
+/// \brief Declares the TargetLoweringMIPS32 class, which implements the
 /// TargetLowering interface for the MIPS 32-bit architecture.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef SUBZERO_SRC_ICETARGETLOWERINGMIPS32_H
 #define SUBZERO_SRC_ICETARGETLOWERINGMIPS32_H
 
 #include "IceDefs.h"
 #include "IceInstMIPS32.h"
 #include "IceRegistersMIPS32.h"
@@ skipping 277 matching lines @@
 protected:
   explicit TargetHeaderMIPS32(GlobalContext *Ctx);
 
 private:
   ~TargetHeaderMIPS32() = default;
 };
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICETARGETLOWERINGMIPS32_H