[lldb] Accept negative indexes in __getitem__

To the Python bindings, add support for Python-like negative indexes.

While was using `script`, I tried to access a thread's bottom frame with
`thread.frame[-1]`, but that failed. This change updates the `__getitem__`
implementations to support negative indexes as one would expect in Python.

Differential Revision: https://reviews.llvm.org/D143282