[lldb] Change implementation of memory read --show-tags option

This does 2 things:
* Moves it after the short options. Which makes sense given it's
  a niche, default off option.
  (if 2 files for one option seems a bit much, I am going to reuse
  them for "memory find" later)
* Fixes the use of repeated commands. For example:
    memory read buf --show-tags
    <shows tags>
    memory read
    <shows tags>

Added tests for the repetition and updated existing help tests.

Reviewed By: omjavaid

Differential Revision: https://reviews.llvm.org/D125089