[UpdateTestChecks] Make generation of UTC_ARGS: comment more robust

We now use the argparse Action objects to determine the name of the flags.
This fixes cases where the key for the stored result ('dest') is not the
same as the command line flag (e.g. --enable/--disable).
Also add a test that --disabled can be part of the initial UTC_ARGS.

This is split out from D78478

Reviewed By: jdoerfert
Differential Revision: https://reviews.llvm.org/D78617