Remove SwiftOnoneSupport hack for implicit opening of existentials

Author: DougGregor

lib/Frontend/CompilerInvocation.cpp

@@ -459,11 +459,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableExperimentalVariadicGenerics |=
     Args.hasArg(OPT_enable_experimental_variadic_generics);
 
-  // SwiftOnoneSupport produces different symbols when opening existentials,
-  // so disable it.
-  if (FrontendOpts.ModuleName == SWIFT_ONONE_SUPPORT)
-    Opts.EnableOpenedExistentialTypes = false;
-
   Opts.EnableExperimentalDistributed |=
     Args.hasArg(OPT_enable_experimental_distributed);