[lldb] Add long help to `crashlog`

Convert the `crashlog` command to be implemented as a class. The `Symbolicate`
function is switched to a class, to implement `get_long_help`. The text for the
long help comes from the help output generated by `OptionParser`. That is, the
output of `help crashlog` is the same as `crashlog --help`.

Differential Revision: https://reviews.llvm.org/D117165