swift234

[stdlib] Revert "Gyb ManagedBuffer..."

This reverts r22541.  It turns out that because of the type-punning we
need in order to handle the canonical empty array buffer in Array<Int>
and Array<SomeClass>, calls through methods that could be
dynamically-dispatched (i.e. methods of classes) can't be used to access
an array buffer.  So, for arrays, we need to keep the HeapBuffer
strategy wherein a struct manager object does all the address
calculations.

ManagedBuffer is still a useful tool for users though, and it fills the
role that HeapBuffer was intended to serve for them better than
HeapBuffer itself does, because it is simpler and more type-safe.

Swift SVN r22573