[Async CC] Save parent executor into context.

When an async function is called from an async function, the caller
stores its own executor into the the callee's context in the
ResumeParentExecutor field.

Author: nate-chandler

lib/IRGen/GenCall.cpp

@@ -2205,6 +2205,12 @@ class AsyncCallEmission final : public CallEmission {
       auto fieldLayout = layout.getParentLayout();
       saveValue(fieldLayout, explosion, isOutlined);
     }
+    { // caller executor
+      Explosion explosion;
+      explosion.add(IGF.getAsyncExecutor());
+      auto fieldLayout = layout.getResumeParentExecutorLayout();
+      saveValue(fieldLayout, explosion, isOutlined);
+    }
     // Move all the arguments into the context.
     for (unsigned index = 0, count = layout.getIndirectReturnCount();
          index < count; ++index) {