Client code can make a best effort at emitting a key path referencing a property with its publicly exposed API, which in the common case will match what the defining module would produce as the canonical key path component representation of the declaration. We can reduce the code size impact of these descriptors by not emitting them when there's no hidden or possibly-resiliently-changed-in-the-past information about a storage declaration, having the property descriptor symbol reference a sentinel value telling client key paths to use their definition of the key path component.
A public subscript might have generic indexes that aren't unconditionally Hashable, or might use indexes that are retroactively made Hashable, so the property descriptor on the implementer's side can't always resiliently provide this information to the final instantiated KeyPath.
