[lldb] Extend Python convenience variable table with equivalent APIs

[lldb] Extend Python convenience variable table with equivalent APIs

Add a column to the table of convenience variables with the equivalent
API to get to the current debugger, target, process, etc.

We often get asked to make convenience variables available outside of
the interactive interpreter. After explaining why that's not possible, a
common complaint is that it's hard to find out how to get to these
variables in a non-interactive context, for example how to get to the
current frame when given a thread. This patch aims to alleviate that by
including the APIs to navigate between these instances in the table.

Differential revision: https://reviews.llvm.org/D97778