opts: Use OPTION_SET_P instead of magic value 2 for -fshort-enums default [PR118011]

The magic values for default (usually -1 or sometimes 2) for some options
are from times we haven't global_options_set, I think we should eventually
get rid of all of those.

The PR is about gcc -Q --help=optimizers reporting -fshort-enums as
[enabled] when it is disabled.
For this the following patch is just partial fix; with explicit
gcc -Q --help=optimizers -fshort-enums
or
gcc -Q --help=optimizers -fno-short-enums
it already worked correctly before, with this patch it will report
even with just
gcc -Q --help=optimizers
correct value on most targets, except 32-bit arm with some options or
defaults, so I think it is a step in the right direction.

But, as I wrote in the PR, process_options isn't done before --help=
and even shouldn't be in its current form where it warns on some option
combinations or errors or emits sorry on others, so I think ideally
process_options should have some bool argument whether it is done for
--help= purposes or not, if yes, not emit warnings and just adjust the
options, otherwise do what it currently does.

2024-12-14  Jakub Jelinek  <jakub@redhat.com>

	PR c/118011
gcc/
	* opts.cc (init_options_struct): Don't set opts->x_flag_short_enums to
	2.
	* toplev.cc (process_options): Test !OPTION_SET_P (flag_short_enums)
	rather than flag_short_enums == 2.
gcc/ada/
	* gcc-interface/misc.cc (gnat_post_options): Test
	!OPTION_SET_P (flag_short_enums) rather than flag_short_enums == 2.