[Async CC] Never map returns to native explosions.

Previously, when lowering the entry point of an async function, the
returned values were lowered to explosions that matched those of sync
functions, namely native explosions. That is incorrect for async
functions where the structured values are within the async context.

Here, that error is fixed, by loading the values returned from the call
out of the async context passed in.

rdar://problem/71641793

Author: nate-chandler

lib/IRGen/GenCall.cpp

@@ -2390,28 +2390,12 @@ class AsyncCallEmission final : public CallEmission {
     }
   }
   void emitCallToUnmappedExplosion(llvm::CallInst *call, Explosion &out) override {
-    SILFunctionConventions fnConv(getCallee().getSubstFunctionType(),
-                                  IGF.getSILModule());
-    auto resultType =
-        fnConv.getSILResultType(IGF.IGM.getMaximalTypeExpansionContext());
-    auto &nativeSchema =
-        IGF.IGM.getTypeInfo(resultType).nativeReturnValueSchema(IGF.IGM);
-    auto expectedNativeResultType = nativeSchema.getExpandedType(IGF.IGM);
-    if (expectedNativeResultType->isVoidTy()) {
-      // If the async return is void, there is no return to move out of the
-      // argument buffer.
-      return;
-    }
-    // Gather the values.
-    Explosion nativeExplosion;
     auto layout = getAsyncContextLayout();
     for (unsigned index = 0, count = layout.getDirectReturnCount();
          index < count; ++index) {
       auto fieldLayout = layout.getDirectReturnLayout(index);
-      loadValue(fieldLayout, nativeExplosion);
+      loadValue(fieldLayout, out);
     }
-
-    out = nativeSchema.mapFromNative(IGF.IGM, IGF, nativeExplosion, resultType);
   }
   Address getCalleeErrorSlot(SILType errorType) override {
     auto layout = getAsyncContextLayout();