swift234

Revert "[stdlib] mask unused bits in uniqueness checks"
Revert "[stdlib] Use an enum for ArrayBuffer storage"

This reverts commit r18996.
This reverts commit r18954.

The optimizer is not ready yet to handle this change especially given
the time until Beta 3. After speaking with DaveA, we agreed to revert
this and take such large changes onto private branches until we are sure
that the optimizer is ready to handle them rather than risking
performance regressions due to hitting the "optimization cliff".

Swift SVN r19026