[TableGen] Add backend to generate command guide for tools using libOption.

For lldb and dsymutil, the command guide is essentially a copy of its
help output generated by libOption. Making sure the two stay in sync is
tedious and error prone. Given that we already generate the help from a
tablegen file, we might as well generate the RST as well.

This adds a tablegen backend for generating Sphinx/RST command guides
from the tablegen file.

Differential revision: https://reviews.llvm.org/D70610