[lldb] Make callback-based formatter matching available from the CLI.

This change adds a `--recognizer-function` (`-R`) to `type summary add`
and `type synth add` that allows users to specify that the names in
the command are not type names but python function names.

It also adds an example to lldb/examples, and a section in the data
formatters documentation on how to use recognizer functions.

Differential Revision: https://reviews.llvm.org/D137000