swift234

ManagedBufferPointer checks whether the buffer class conforms to certain constraints
and whether the minimum capacity is greater than 0

These checks are not neccessary for its user _ContigousArrayBuffer where we know
that they will hold true.

Use private init functions for _ContigousArrayBuffer instead that don't perform
those checks.

PerfTests_O results:
ArrayLiteral +50%
Forest +20%

Several ~10% improvments under PerfTests_Onone.

rdar://19782983

Swift SVN r25141