Add a frontend option to enable parsing "operator protocol designators".

Author: rudkx

include/swift/Basic/LangOptions.h

@@ -203,6 +203,9 @@ namespace swift {
     /// \brief Enable experimental property behavior feature.
     bool EnableExperimentalPropertyBehaviors = false;
 
+    /// \brief Enable experimental operator protocol designator feature.
+    bool EnableOperatorDesignatedProtocols = false;
+
     /// \brief Staging flag for treating inout parameters as Thread Sanitizer
     /// accesses.
     bool DisableTsanInoutInstrumentation = false;

include/swift/Option/FrontendOptions.td

@@ -309,6 +309,10 @@ def enable_experimental_property_behaviors :
   Flag<["-"], "enable-experimental-property-behaviors">,
   HelpText<"Enable experimental property behaviors">;
 
+def enable_operator_designated_protocols :
+  Flag<["-"], "enable-operator-designated-protocols">,
+  HelpText<"Enable operator designated protocols">;
+
 def enable_deserialization_recovery :
   Flag<["-"], "enable-deserialization-recovery">,
   HelpText<"Attempt to recover from missing xrefs (etc) in swiftmodules">;

lib/Frontend/CompilerInvocation.cpp

@@ -161,6 +161,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableExperimentalPropertyBehaviors |=
     Args.hasArg(OPT_enable_experimental_property_behaviors);
 
+  Opts.EnableOperatorDesignatedProtocols |=
+      Args.hasArg(OPT_enable_operator_designated_protocols);
+
   if (auto A = Args.getLastArg(OPT_enable_deserialization_recovery,
                                OPT_disable_deserialization_recovery)) {
     Opts.EnableDeserializationRecovery