[gn build] revert e8e078c8bf

Now that I've updated ancient goma clients on the bots, this should
work. (Internal goma bug: b/139410332, fixed months ago.)

llvm/utils/gn/build/BUILD.gn

@@ -196,9 +196,7 @@ config("compiler_defaults") {
     }
     if (use_relative_paths_in_debug_info) {
       cflags += [
-        "-Xclang",
         "-fdebug-compilation-dir",
-        "-Xclang",
         ".",
       ]
     }