swift234

[stdlib] Refactor String.UTF8Index for conversion

The old design did not strictly keep track of the index in underlying
UTF16, which would have made converting between the different index
types too difficult.  It also made equality comparison between indices
broken, because

  UTF8Index(s.utf16.startIndex+1, within: s.utf8)

and

  UTF8Index(s.utf16.startIndex, within: s.utf8).successor()

would often have completely different UTF8 buffers and offsets within
the underlying UTF16.

For some reason this disturbed SILPasses/devirt_default_case.swift,
which is now XFAIL'd.  <rdar://problem/19298212>
SILPasses/devirt_default_case.swift is XFAIL'd

Swift SVN r24012