Driver passes -O1 by default

In Clang::ConstructJob:
+  // For FLANG, we want to pass -O1 as the default, iff no -O was given
+  if (IsFlang && (!Args.getLastArg(options::OPT_O_Group)))
+      CmdArgs.push_back("-O1");

Shouldn't do that to make sure debugging experience is good.  Let the user choose the opt level.