swift234

[stdlib] Replace String with NewString

This change is as minimal as possible, which means leaving obsolete
functionality in place (e.g. StringByteData) and even keeping the
"NewString" name in many places.  The obsolete functionality was useful
for testing my changes, but expect immediate cleanup commits
addressing all those issues to follow.

* All String bridging now happens in pure Swift code.

* Because String no longer owns an array of UInt8, some assumptions and
  assertions are no longer valid.  As a result, some code was deleted
  and all the code that produces null-terminated strings had to be
  rewritten

* test/Constraints/construction.swift had to have one test commented out
  because it relied on an element of the String interface that I did not
  port forward.  It seems to me that this test should declare its own
  types and not rely on the stdlib, if it's still valid.

* One test in /test/stdlib/Algorithm.swift had to be disabled pending
  <rdar://problem/15736729> and <rdar://problem/15733855>

* This change revealed that test/Interpreter/repl.swift is sensitive to
  type-alias names; I had to change a "NewString" to "String" there.
  This may indicate a bug somewhere?

Swift SVN r11830