[lldb] Print the enum values and their description in the help output

Print the enum values and their description in the help output for
argument values. Until now, there was no way to get these values and
their description.

Example output:

(lldb) help <description-verbosity>
  <description-verbosity> -- How verbose the output of 'po' should be.

     compact : Only show the description string
     full    : Show the full output, including persistent variable's
               name and type

Differential revision: https://reviews.llvm.org/D129707