[Bug] incorrect invalid return type error using shapes and union type

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**Describe the bug**

With the union type feature enabled, hh_client doesn't seem to understand the union between shapes that contain optional extra fileds.

**Standalone code, or other way to reproduce the problem**
```hack
interface C {
public function with(): C<(T & Ts)>;
}

function bar(
C shape('a' => int), ...)> $config,
): C shape('a' => int), 'bar' => int, ...)> {
return $config->with int,
...
)>();
}
```

**Expected behavior**

No errors.

**Actual behavior**

```
Typing[4110] Invalid return typeHack(4110)
playground.hack(20, 6): Expected shape('bar' => int, 'hello' => shape('a' => int), ...)
playground.hack(20, 6): This type argument to Nuxed\Playground\C must match exactly (it is invariant)
playground.hack(15, 39): But got (shape('hello' => shape('a' => int), ...) & shape('bar' => int, ...))
```

**Environment**

```
❯ hhvm --version
HipHop VM 4.85.0 (rel)
Compiler: 1606769234_148788027
Repo schema: d1ae8e21bf3419a65f12a010527485564e719d07
❯ hh_client --version
hackc-fd31045126d9e76368e3fa73817029cf8e5da3f3-4.85.0
❯ hh_server --version
hackc-fd31045126d9e76368e3fa73817029cf8e5da3f3-4.85.0
```

Reproduce the standalone Hack example with union types using hh_client and compare the reported invalid return type with the expected shape type. Use the diagnostics from playground.hack as the starting reference, then verify the behavior against the listed HHVM, hh_client, and hh_server versions. Done means the example produces no invalid return type error.

Written by the indexing model from the issue text.