[NFC] Remove remnants of the old -sil-existential-specializer option.

This option no longer works but was still in the option file and help
message.

Add a temporary pass-specific option instead for debugging until this
pass has been sufficiently tested yet.

lib/Frontend/CompilerInvocation.cpp

@@ -702,9 +702,6 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
       return true;
     }
   }
-  if (Args.hasArg(OPT_sil_existential_specializer)) {
-    Opts.ExistentialSpecializer = true;
-  }
   if (const Arg *A = Args.getLastArg(OPT_num_threads)) {
     if (StringRef(A->getValue()).getAsInteger(10, Opts.NumThreads)) {
       Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,