Sema: Downgrade potentially unavailable enum cases to a warning in module interfaces

There are two pieces here:

- A -warn-on-potentially-unavailable-enum-case flag is passed down by
  the driver when *producing* a swiftinterface

- When *consuming* a swiftinterface, also enable this behavior

Part of rdar://problem/78306593.

lib/Frontend/CompilerInvocation.cpp

@@ -457,6 +457,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
       = A->getOption().matches(OPT_enable_conformance_availability_errors);
   }
 
+  Opts.WarnOnPotentiallyUnavailableEnumCase |=
+      Args.hasArg(OPT_warn_on_potentially_unavailable_enum_case);
   if (auto A = Args.getLastArg(OPT_enable_access_control,
                                OPT_disable_access_control)) {
     Opts.EnableAccessControl