[Frontend] Add flag to enable new operator lookup

For now disabled by default, but once we get a new
language mode, we should flip it on by default.

lib/Frontend/CompilerInvocation.cpp

@@ -409,6 +409,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.StressASTScopeLookup |= Args.hasArg(OPT_stress_astscope_lookup);
   Opts.WarnIfASTScopeLookup |= Args.hasArg(OPT_warn_if_astscope_lookup);
   Opts.LazyASTScopes |= Args.hasArg(OPT_lazy_astscopes);
+  Opts.EnableNewOperatorLookup = Args.hasFlag(OPT_enable_new_operator_lookup,
+                                              OPT_disable_new_operator_lookup,
+                                              /*default*/ false);
   Opts.UseClangFunctionTypes |= Args.hasArg(OPT_use_clang_function_types);
 
   Opts.NamedLazyMemberLoading &= !Args.hasArg(OPT_disable_named_lazy_member_loading);