We end up with uninit nulls in the repo for unit constants.
In RepoAuthoritative mode, we will use m_type == KindOfUninit during
constant lookup to decide whether to cast TypedValue::m_data.pref to a
ConstantInfo*, and then call a function pointer that it contains. (We
should probably also clean that up to not cast RefData*'s to
ConstantInfo*'s, but I left that out of this diff.)
Jordan DeLong