[Freestanding] Use task-to-thread concurrency model.

Defined SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY to describe
whether the standard library will use the task-to-thread model for
concurrency.  It is true only for freestanding non-Darwin stdlibs.

When it is true, SWIFT_CONCURRENCY_TASK_TO_THREAD_MODEL is defined
during stdlib compilation of both Swift and C++ sources.

Added an option to LangOptions to specify which concurrency model is
used, either standard or task-to-thread.  When
SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY is true, the model is
specified to be task-to-thread.

lib/Frontend/CompilerInvocation.cpp

@@ -1018,6 +1018,14 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.DumpTypeWitnessSystems = Args.hasArg(OPT_dump_type_witness_systems);
 
+  if (const Arg *A = Args.getLastArg(options::OPT_concurrency_model)) {
+    Opts.ActiveConcurrencyModel =
+        llvm::StringSwitch<ConcurrencyModel>(A->getValue())
+            .Case("standard", ConcurrencyModel::Standard)
+            .Case("task-to-thread", ConcurrencyModel::TaskToThread)
+            .Default(ConcurrencyModel::Standard);
+  }
+
   return HadError || UnsupportedOS || UnsupportedArch;
 }