Use &_dispatch_main_q as the identity of the main actor.

I added Builtin.buildMainActorExecutor before, but because I never
implemented it correctly in IRGen, it's not okay to use it on old
versions, so I had to introduce a new feature only for it.

The shim dispatch queue class in the Concurrency runtime is rather
awful, but I couldn't think of a reasonable alternative without
just entirely hard-coding the witness table in the runtime.
It's not ABI, at least.

lib/AST/ASTPrinter.cpp

@@ -2768,6 +2768,10 @@ static bool usesFeatureBuiltinBuildExecutor(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureBuiltinBuildMainExecutor(Decl *decl) {
+  return false;
+}
+
 static bool usesFeatureBuiltinContinuation(Decl *decl) {
   return false;
 }