[llvm][TableGen] Define FieldInit::isConcrete overload

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