[PR92726, PR92970, PR92984] [OpenACC] Clarify 'acc_delete' etc. for 'NULL'-in, non-present data, or size zero

PR92970 "OpenACC 2.5: 'acc_delete' etc. on non-present data is a no-op" is an
actual bug fix, and the other ones are fall-out, currently undefined behavior.

	libgomp/
	PR libgomp/92726
	PR libgomp/92970
	PR libgomp/92984
	* oacc-mem.c (delete_copyout): No-op behavior if 'lookup_host'
	fails.
	(GOACC_enter_exit_data): Simplify accordingly.
	* testsuite/libgomp.oacc-c-c++-common/pr92970-1.c: New file,
	subsuming...
	* testsuite/libgomp.oacc-c-c++-common/lib-17.c: ... this file...
	* testsuite/libgomp.oacc-c-c++-common/lib-18.c: ..., and this
	file.
	* testsuite/libgomp.oacc-c-c++-common/pr92984-1.c: New file,
	subsuming...
	* testsuite/libgomp.oacc-c-c++-common/lib-21.c: ... this file...
	* testsuite/libgomp.oacc-c-c++-common/lib-29.c: ..., and this
	file.
	* testsuite/libgomp.oacc-c-c++-common/pr92726-1.c: New file,
	subsuming...
	* testsuite/libgomp.oacc-c-c++-common/lib-28.c: ... this file.

From-SVN: r279532