[lldb] Add `source cache dump` and `source cache clear` subcommand

Add two new source subcommands: source cache dump and source cache
clear. As the name implies the first one dumps the source cache while
the later clears the cache.

This patch was motivated by a handful of (internal) bug reports related
to sources not being available. Right now those issues can be hard to
diagnose. The new commands give users, as well as us as developers, more
insight into and control over the source cache.

Differential revision: https://reviews.llvm.org/D153685