[stdlib] String.Index: Add _description & _debugDescription
[stdlib] String.Index: Add _description & _debugDescription

While we’re vacillating on how best to add CustomStringConvertible
and CustomDebugStringConvertible conformances to String.Index, add
the prospective implementations as underscored-but-public members,
to make life a little more bearable for people who need to debug
string index operations.

roambotics/swift,roambotics/swift,roambotics/swift,apple/swift,roambotics/swift,roambotics/swift,apple/swift,apple/swift,apple/swift,roambotics/swift,apple/swift,apple/swift