[ Typechecker ] Message when passing A where B is expected when A and B are newtypes does not mention A, just ?nonnull

**Describe the bug**
Message when passing `KindOfHerp` where `KindOfDerp` is expected when `KindOfHerp` and `KindOfDerp` are newtypes does not mention `KindOfHerp`. Instead it says that the newtype is constrained to `?nonnull` a.k.a. `mixed`.

**Standalone code, or other way to reproduce the problem**

```HACK
// file1.hack
newtype KindOfHerp = string;
newtype KindOfDerp = string;

function herp(): KindOfHerp {
return 'KindOfHerp';
}

// file2.hack
function derp(): KindOfDerp {
return herp();
}
```

Steps to reproduce the behavior:
1. hh_client

**Expected behavior**

```
Typing[4110] Invalid return type [1]
-> Expected KindOfDerp [2]
-> But got KindOfHerp [3]
```

**Actual behavior**

```
Typing[4110] Invalid return type [1]
-> Expected KindOfDerp [2]
-> But got mixed arising from an implicit as ?nonnull constraint on this type [3]
```

**Environment**
- Operating system
> Ubuntu 18.04
- Installation method
> apt-get with dl.hhvm.com repository
- HHVM Version
```
HipHop VM 4.107.0 (rel)
Compiler: 1619546863_111004381
Repo schema: d1ae8e21bf3419a65f12a010527485564e719d07
hackc-c7fffd36e382b7f28420653e7a0f0e78e61342a8-4.107.0
```

**Additional context**

The full error message, annotated with `^^^^` in highlighted parts of the error.
```
Typing[4110] Invalid return type [1]
-> Expected KindOfDerp [2]
-> But got mixed arising from an implicit as ?nonnull constraint on this type [3]

file1.hack:-:-
|
[2] | function derp(): KindOfDerp {
^^^^^^^^^^
[1] | return herp();
^^^^^^
| }

file2.hack:-:-
| newtype KindOfDerp = string;
|
[3] | function herp(): KindOfHerp {
^^^^^^^^^^
| return 'KindOfHerp';
| }

1 error found.
```