With the introduction of `next(isolation:)` into the task group types,
we removed the public APIs for the no-argument `next()` versions,
leaving them only as `@usableFromInline internal` entrypoints for the
ABI. Because the `next(isolation:)` versions have a default argument,
this was sufficient for providing source compatibility for calls, but
not for protocol conformances.
Bring these APIs back publicly. Fixes rdar://127499568.
