SILGen: Implement native-to-foreign thunks for async methods.

Bridging an async Swift method back to an ObjC completion-handler-based API requires
that the ObjC thunk spawn a task on which to execute the Swift async API and pass
its results back on to the completion handler.

lib/SIL/IR/SILDeclRef.cpp

@@ -1230,3 +1230,13 @@ bool SILDeclRef::isDynamicallyReplaceable() const {
   // enabled.
   return decl->shouldUseNativeMethodReplacement();
 }
+
+bool SILDeclRef::hasAsync() const {
+  if (hasDecl()) {
+    if (auto afd = dyn_cast<AbstractFunctionDecl>(getDecl())) {
+      return afd->hasAsync();
+    }
+    return false;
+  }
+  return getAbstractClosureExpr()->isBodyAsync();
+}