OpenMP: Fix regressions in metadirective-target-device-2.c [PR120518]

OpenMP: Fix regressions in metadirective-target-device-2.c [PR120518]

My previous patch that added a CLEANUP_POINT_EXPR around the device_num
selector expression in the C++ front end broke the testcase
c-c++-common/gomp/metadirective-target-device-2.c on offload targets.
It confused the code in omp_device_num_check that tries to bypass error
checking and do early resolution when the expression is a call to one
of the OpenMP library functions.  The solution is to make that code smart
enough to look inside a CLEANUP_POINT_EXPR.

gcc/ChangeLog
	PR c++/120518
	* omp-general.cc (omp_device_num_check): Look inside a
	CLEANUP_POINT_EXPR when trying to optimize special cases.