[OrderedSet] Don't let the unchecked init create large sets with no hash table

[lorentey]

In debug builds, the OrderedSet.init(uncheckedUniqueElements:) initializer always created a set with no hash table, even if it had an element count >15. This is technically an invariant violation (caught by _checkInvariants()), although it generally only triggers (very) slow performance. (The set reverts to linear searching.)

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[lorentey]

(This was uncovered while trying to reproduce #104; it's a real bug, but sadly it's a performance issue, not a correctness bug like that one.)

[lorentey]

@swift-ci test

[lorentey]

Rebased on top of latest changes on release/1.0 to satisfy branch protection rules.

@swift-ci test