[Concurrency] Remove -executor-factory option and replace with magic type.

We decided that using a magic typealias to set the executor factory was better
than using a compiler option. Remove the `-executor-factory` option, and replace
by looking up the `DefaultExecutorFactory` type, first in the main module, and
then if that fails in Concurrency.

rdar://149058236

lib/Frontend/CompilerInvocation.cpp

@@ -1364,12 +1364,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.enableFeature(Feature::RegionBasedIsolation);
   }
 
-  // Get the executor factory name
-  if (const Arg *A = Args.getLastArg(OPT_executor_factory)) {
-    printf("Got executor-factory option\n");
-    Opts.ExecutorFactory = A->getValue();
-  }
-
   Opts.WarnImplicitOverrides =
     Args.hasArg(OPT_warn_implicit_overrides);