[clang] fix a compilation bug

With gcc 6.3.0, I hit the following compilation bug:
  /home/yhs/work/llvm-project/clang/lib/Frontend/CompilerInvocation.cpp:
  In function ‘bool ParseCodeGenArgs(clang::CodeGenOptions&, llvm::opt::ArgList&,
  clang::InputKind, clang::DiagnosticsEngine&, const clang::TargetOptions&,
  const clang::FrontendOptions&)’:
  /home/yhs/work/llvm-project/clang/lib/Frontend/CompilerInvocation.cpp:780:12:
    error: unused variable ‘A’ [-Werror=unused-variable]
     if (Arg *A = Args.getLastArg(OPT_fuse_ctor_homing))
              ^
  cc1plus: all warnings being treated as errors

The bug is introduced by Commit ae6523cd ("[DebugInfo] Add
-fuse-ctor-homing cc1 flag so we can turn on constructor homing only
if limited debug info is already on.")