[ TypeChecker ] Shapes::at() does not generate an error when a field is known to not exist if the given key is a class constant

**Describe the bug**
Indexing into a shape using `Shapes::at($shape, SomeClass::SOME_CONSTANT)` silently returns `nothing` instead of emitting a typechecker error.

**Standalone code, or other way to reproduce the problem**
```HACK
final class MyClass {
const string A = 'A';
const string B = 'B';
}

function shape_with_class_const_fields(): shape(MyClass::A => string) {
return shape(MyClass::A => 'A');
}

function shape_with_plain_string_fields(): shape('A' => string) {
return shape('A' => 'A');
}

function indexing_into_shape(): void {
$_strange_since_myclass_b_is_known_to_not_exist = Shapes::at(shape_with_class_const_fields(), MyClass::B);
$_even_stranger_since_myclass_is_not_related_to_this_shape = Shapes::at(shape_with_plain_string_fields(), MyClass::B);

// Typing[4251] You are calling Shapes::at() on a field known to not exist Hack(4251)
// file.hack(6, 43): The field A is not defined in this shape
$_expected_this_shape_has_no_plain_string_keys = Shapes::at(shape_with_class_const_fields(), 'A');
// Typing[4251] You are calling Shapes::at() on a field known to not exist Hack(4251)
// file.hack(5, 44): The field B is not defined in this shape
$_expected_this_shape_has_string_a_but_not_b = Shapes::at(shape_with_plain_string_fields(), 'B');
}
```
Steps to reproduce the behavior:
1. hh_client

**Expected behavior**

All these cases should result in an error.

**Actual behavior**

Indexing with a class constant is not a typechecker error. At runtime this throws.

Copy-paste output, or add a screenshot to illustrate what actually happens. Copy-pasted text output (e.g. from `hhvm` or `hh_client`) is preferred to screenshots.

```
Typing[4251] You are calling Shapes::at() on a field known to not exist [1]
-> The field A is not defined in this shape [2]

file.hack:20:54
4 | }
5 |
[2] 6 | function shape_with_class_const_fields(): shape(MyClass::A => string) {
7 | return shape(MyClass::A => 'A');
8 | }
:
18 | // Typing[4251] You are calling Shapes::at() on a field known to not exist Hack(4251)
19 | // file.hack(6, 43): The field A is not defined in this shape
[1] 20 | $_expected_this_shape_has_no_plain_string_keys = Shapes::at(shape_with_class_const_fields(), 'A');
21 | // Typing[4251] You are calling Shapes::at() on a field known to not exist Hack(4251)
22 | // file.hack(5, 44): The field B is not defined in this shape

Typing[4251] You are calling Shapes::at() on a field known to not exist [1]
-> The field B is not defined in this shape [2]

file.hack:23:52
8 | }
9 |
[2] 10 | function shape_with_plain_string_fields(): shape('A' => string) {
11 | return shape('A' => 'A');
12 | }
:
21 | // Typing[4251] You are calling Shapes::at() on a field known to not exist Hack(4251)
22 | // file.hack(5, 44): The field B is not defined in this shape
[1] 23 | $_expected_this_shape_has_string_a_but_not_b = Shapes::at(shape_with_plain_string_fields(), 'B');
24 | }

2 errors found.
```

**Environment**
- Operating system
> 'Ubuntu 18.04'
- Installation method
> 'hhvm/hhvm on dockerhub'
- HHVM Version
```
HipHop VM 4.121.0 (rel) (non-lowptr)
Compiler: 1627924236_151424549
Repo schema: 10a34637ad661d98ba3344717656fcc76209c2f8
hackc-d5e69b1378e833da7a831b4ced2e3f28824d6fad-4.121.0
```

**Additional context**
We use class constant shapes if the field name needs to be changeable in the future. Mostly at API boundaries. These kinds of shapes have a lot of optional fields, which invites the use of `Shapes::at()`. This makes not having the typechecker correct us here extra difficult to deal with.