[Sema] Fix warnings

This patch fixes:

  clang/lib/Sema/SemaHLSL.cpp:838:12: error: unused variable
  'TheVarDecl' [-Werror,-Wunused-variable]

  clang/lib/Sema/SemaHLSL.cpp:840:19: error: unused variable
  'CBufferOrTBuffer' [-Werror,-Wunused-variable]

Author: kazutakahirata

clang/lib/Sema/SemaHLSL.cpp

@@ -843,6 +843,8 @@ static void DiagnoseHLSLRegisterAttribute(Sema &S, SourceLocation &ArgLoc,
   assert(((TheVarDecl && !CBufferOrTBuffer) ||
           (!TheVarDecl && CBufferOrTBuffer)) &&
          "either TheVarDecl or CBufferOrTBuffer should be set");
+  (void)TheVarDecl;
+  (void)CBufferOrTBuffer;
 
   RegisterBindingFlags Flags = HLSLFillRegisterBindingFlags(S, TheDecl);
   assert((int)Flags.Other + (int)Flags.Resource + (int)Flags.Basic +