Make 'c-c++-common/goacc/kernels-decompose-pr100400-1-*.c' behave consistently, regar...

Make 'c-c++-common/goacc/kernels-decompose-pr100400-1-*.c' behave consistently, regardless of checking level

Fix-up for commit c14ea6a72fb1ae66e3d32ac8329558497c6e4403
"Catch 'GIMPLE_DEBUG' misbehavior in OpenACC 'kernels' decomposition
[PR100400, PR103836, PR104061]".

For C++ compilation of 'c-c++-common/goacc/kernels-decompose-pr100400-1-2.c',
we first emit a 'sorry' diagnostic, and then a 'gcc_unreachable' (or
'internal_error', see below) diagnostic, but for example, for
'--enable-checking=release' (thus, '!CHECKING_P'), the second one may actually
be turned into a 'confused by earlier errors, bailing out' diagnostic.  (See
'gcc/diagnostic.cc:diagnostic_report_diagnostic': "When not checking, ICEs are
converted to fatal errors when an error has already occurred.")  Thus, make
'c-c++-common/goacc/kernels-decompose-pr100400-1-2.c' behave consistently via
'-Wfatal-errors', and thus only matching the 'sorry' diagnostic.

For example, for '--enable-checking=no' (thus, '!ENABLE_ASSERT_CHECKING'), a
call to 'gcc_unreachable' cannot be assumed emit an 'internal_error'-like
diagnostic, so explicitly call 'internal_error' in
'gcc/omp-oacc-kernels-decompose.cc:visit_loops_in_gang_single_region', in the
'GIMPLE_OMP_FOR' case, to avoid regressing
'c-c++-common/goacc/kernels-decompose-pr100400-1-3.c', and
'c-c++-common/goacc/kernels-decompose-pr100400-1-4.c'.

	PR middle-end/100400
	gcc/
	* omp-oacc-kernels-decompose.cc
	(visit_loops_in_gang_single_region) <GIMPLE_OMP_FOR>: Explicitly
	call 'internal_error'.
	gcc/testsuite/
	* c-c++-common/goacc/kernels-decompose-pr100400-1-2.c: Specify
	'-Wfatal-errors'.