[clang-linker-wrapper] Fix a warning

This patch fixes:

  clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp:642:6:
  error: unused function 'diagnosticHandler'
  [-Werror,-Wunused-function]

Author: kazutakahirata

clang/tools/clang-linker-wrapper/ClangLinkerWrapper.cpp

@@ -639,28 +639,6 @@ Expected<StringRef> linkDevice(ArrayRef<StringRef> InputFiles,
   }
 }
 
-void diagnosticHandler(const DiagnosticInfo &DI) {
-  std::string ErrStorage;
-  raw_string_ostream OS(ErrStorage);
-  DiagnosticPrinterRawOStream DP(OS);
-  DI.print(DP);
-
-  switch (DI.getSeverity()) {
-  case DS_Error:
-    WithColor::error(errs(), LinkerExecutable) << ErrStorage << "\n";
-    break;
-  case DS_Warning:
-    WithColor::warning(errs(), LinkerExecutable) << ErrStorage << "\n";
-    break;
-  case DS_Note:
-    WithColor::note(errs(), LinkerExecutable) << ErrStorage << "\n";
-    break;
-  case DS_Remark:
-    WithColor::remark(errs()) << ErrStorage << "\n";
-    break;
-  }
-}
-
 Expected<StringRef> writeOffloadFile(const OffloadFile &File) {
   const OffloadBinary &Binary = *File.getBinary();