[llvm-dwarfdump] Help option output should be consistent with the command guide

The dwarfdump command guide shows the short options used as aliases but
these are not found in the help text unless --show-hidden is used.
Investigating other tools some follow this pattern, others like
llvm-objdump show aliases with --help. This change fixes the help output
to be consistent with the command guide. This includes updating alias
descriptions in the help output to use "--".

As part of this change I updated cmdline.test, including some options
that were missing testing.

Differential Revision: https://reviews.llvm.org/D101646