swift234

Review: Index protocol extensions

- Add Strict/Defaulted Index types to StdlibUnittest
- Test whether a random access index calls its more efficient
  customization by tracking successor calls.
- Fix the RandomAccessIndex.advancedBy(n, limit:) API by de-underscoring
  the limit parameter
- Inline some internal transparent default implementations to their only
  call site
- Attach _RandomAccessAmbiguity type to RandomAccessIndex

rdar://problem/22085119

Swift SVN r30979