Crash on unwrapping `_kvcKeyPathString` in `expression<R, V>(for keyPath: KeyPath<R, V>)`

I am hitting this crash on line 4 in KeyPath.swift. I guess we can assume the key path is valid or it would not have compiled so this is coming out of Swift.AnyKeyPath. I know there are issues reported out there about it, but I'm not sure what the solution is. Looking into it some more.

## Links

https://bugs.swift.org/browse/SR-5220
https://bugs.swift.org/browse/SR-6270