swift234

[stdlib] Mirror: Tweaks in response to API review

* Use "reflecting:" rather than "reflect:" as an argument label.  It
  reads more sensibly.

* Unify toString()/toDebugString() with other reflection APIs by converting
  them into String initalizers, and by renaming Printable/DebugPrintable to
  CustomStringConvertible and CustomDebugStringConvertible.

* Drop "make" prefix from "makeCustomMirror" and
  "makeCustomPlaygroundQuickLook".  It is not, as we had thought,
  mandated by Cocoa naming guidelines.

Swift SVN r26918