Summary:
There are a few field init values that are concrete but not complete/foldable (e.g. `?`). This allows for using those values as initializers without erroring out.
Example:
```
class A {
string value = ?;
}
class B<A impl> : A {
let value = impl.value; // This currently emits an error.
let value = ?; // This doesn't emit an error.
}
```
Differential Revision: https://reviews.llvm.org/D74360
River Riddle