[OpenACC privatization] Don't let unhandled 'IFN_UNIQUE_OACC_PRIVATE' linger [PR90115]

[OpenACC privatization] Don't let unhandled 'IFN_UNIQUE_OACC_PRIVATE' linger [PR90115]

Make sure they're all handled in 'execute_oacc_device_lower'.  Why we at all
can have 'level == -1' cases is a separate bug to be fixed.

	gcc/
	PR middle-end/90115
	* omp-offload.c (execute_oacc_device_lower)
	<IFN_UNIQUE_OACC_PRIVATE>: Diagnose and handle for 'level == -1'
	case, too.
	* internal-fn.c (expand_UNIQUE): Don't expect
	'IFN_UNIQUE_OACC_PRIVATE'.