Add "disable" counterpart to -enable-experimental-opened-existential-types

Author: DougGregor

include/swift/Option/FrontendOptions.td

@@ -518,6 +518,10 @@ def enable_experimental_opened_existential_types :
   Flag<["-"], "enable-experimental-opened-existential-types">,
   HelpText<"Enable experimental support for implicitly opened existentials">;
 
+def disable_experimental_opened_existential_types :
+  Flag<["-"], "disble-experimental-opened-existential-types">,
+  HelpText<"Disable experimental support for implicitly opened existentials">;
+
 def enable_deserialization_recovery :
   Flag<["-"], "enable-deserialization-recovery">,
   HelpText<"Attempt to recover from missing xrefs (etc) in swiftmodules">;

lib/Frontend/CompilerInvocation.cpp

@@ -448,8 +448,10 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableParameterizedProtocolTypes |=
       Args.hasArg(OPT_enable_parameterized_protocol_types);
 
-  Opts.EnableOpenedExistentialTypes |=
-      Args.hasArg(OPT_enable_experimental_opened_existential_types);
+  Opts.EnableOpenedExistentialTypes =
+    Args.hasFlag(OPT_enable_experimental_opened_existential_types,
+                 OPT_disable_experimental_opened_existential_types,
+                 false);
 
   // SwiftOnoneSupport produces different symbols when opening existentials,
   // so disable it.