[ELF] openAuxiliaryFile: open /dev/null if disableOutput and filename is "-"

So that LLD_IN_TEST=2 ld.lld --print-archive-stats=- a.o (and -Map -)
only writes the output once.

Author: MaskRay

lld/ELF/Driver.cpp

@@ -105,6 +105,13 @@ llvm::raw_fd_ostream Ctx::openAuxiliaryFile(llvm::StringRef filename,
   using namespace llvm::sys::fs;
   OpenFlags flags =
       auxiliaryFiles.insert(filename).second ? OF_None : OF_Append;
+  if (e.disableOutput && filename == "-") {
+#ifdef _WIN32
+    filename = "NUL";
+#else
+    filename = "/dev/null";
+#endif
+  }
   return {filename, ec, flags};
 }