[Async CC] Save parent context into child context.

When an async function is called from an async function, it stores its
own context pointer into the callee's callerContext.

Author: nate-chandler

lib/IRGen/GenCall.cpp

@@ -2196,9 +2196,16 @@ class AsyncCallEmission final : public CallEmission {
     super::setArgs(asyncExplosion, false, witnessMetadata);
     SILFunctionConventions fnConv(getCallee().getSubstFunctionType(),
                                   IGF.getSILModule());
+    auto layout = getAsyncContextLayout();
 
+    // Set caller info into the context.
+    { // caller context
+      Explosion explosion;
+      explosion.add(IGF.getAsyncContext());
+      auto fieldLayout = layout.getParentLayout();
+      saveValue(fieldLayout, explosion, isOutlined);
+    }
     // Move all the arguments into the context.
-    auto layout = getAsyncContextLayout();
     for (unsigned index = 0, count = layout.getIndirectReturnCount();
          index < count; ++index) {
       auto fieldLayout = layout.getIndirectReturnLayout(index);