[lldb] Adjust libc++ string formatter for changes in D123580

[lldb] Adjust libc++ string formatter for changes in D123580

The code needs more TLC, but for now I've tried making only the changes
that are necessary to get the tests passing -- postponing the more
invasive changes after I create a more comprehensive test.

In a couple of places I have changed the index-based element accesses to
name-based ones (as these are less sensitive to code perturbations). I'm
not sure why the code was using indexes in the first place, but I've
(manually) tested the change with various libc++ versions, and found no
issues with this approach.

Differential Revision: https://reviews.llvm.org/D124113