Depends on D19786. Ref T13217. I had imagined making these new conversions strict, but a lot of stuff breaks when we do because they make sense to use in base classes like CursorPagedPolicyAwareQuery, and it's hard to completely fix all the downstream classes in one fell swoop.

Instead, let them warn for now (like %Q) and we can make them stricter in a future change.

In Phabricator, saw warnings instead of immediate fatals when strings are passed to these conversions.