[lldb] Port lldb gdb-server to libOption

The existing help text was very terse and was missing several important
options. In the new version, I add a short description of each option
and a slightly longer description of the tool as a whole.

The new option list does not include undocumented no-op options:
--debug and --verbose. It also does not include undocumented short
aliases for long options, with two exceptions: -h, because it's
well-known; and -S (--setsid), as it's used in one test. Using these
options will now produce an error. I believe that is acceptable as users
aren't generally invoking lldb-server directly, and the only way to
learn about the short aliases was by looking at the source.

Differential Revision: https://reviews.llvm.org/D89477