[Concurrency] More work on the custom executor implementation.

Added an `-executor-factory` argument to the compiler to let you safely
specify the executors you wish to use (by naming a type that returns
them).

Also added some tests of the new functionality.

rdar://141348916

lib/Frontend/CompilerInvocation.cpp

@@ -1314,6 +1314,12 @@ 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);