[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.

Author: hamishknight

include/swift/Basic/LangOptions.h

@@ -267,6 +267,10 @@ namespace swift {
     /// Build the ASTScope tree lazily
     bool LazyASTScopes = true;
 
+    /// Whether to enable the new operator decl and precedencegroup lookup
+    /// behavior. This is a staging flag, and will be removed in the future.
+    bool EnableNewOperatorLookup = false;
+
     /// Use Clang function types for computing canonical types.
     /// If this option is false, the clang function types will still be computed
     /// but will not be used for checking type equality.

include/swift/Option/FrontendOptions.td

@@ -483,6 +483,14 @@ def disable_invalid_ephemeralness_as_error :
 def switch_checking_invocation_threshold_EQ : Joined<["-"],
     "switch-checking-invocation-threshold=">;
 
+def enable_new_operator_lookup :
+  Flag<["-"], "enable-new-operator-lookup">,
+  HelpText<"Enable the new operator decl and precedencegroup lookup behavior">;
+
+def disable_new_operator_lookup :
+  Flag<["-"], "disable-new-operator-lookup">,
+  HelpText<"Disable the new operator decl and precedencegroup lookup behavior">;
+
 def enable_source_import : Flag<["-"], "enable-source-import">,
   HelpText<"Enable importing of Swift source files">;
 

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);