Fix OpenACC "ephemeral" asynchronous host-to-device copies

This patch fixes several places in libgomp/target.c where "ephemeral" data
(on the stack or in temporary heap locations) may be used as the source of
an asynchronous host-to-device copy that may not complete before the host
data disappears.

An existing, but flawed, workaround for this problem in the AMD GCN
libgomp offloading plugin is currently present on mainline, and was
posted for the og9 branch here:

  https://gcc.gnu.org/legacy-ml/gcc-patches/2019-08/msg00901.html

and previous versions of this patch were posted here (for mainline/og9):

  https://gcc.gnu.org/legacy-ml/gcc-patches/2019-11/msg01482.html
  https://gcc.gnu.org/legacy-ml/gcc-patches/2019-09/msg01026.html

libgomp/
	* libgomp.h (gomp_copy_host2dev): Update prototype.
	* oacc-mem.c (memcpy_tofrom_device, update_dev_host): Add new
	argument to gomp_copy_host2dev (false).
	* plugin/plugin-gcn.c (struct copy_data): Remove free_src field.
	(copy_data): Don't free src.
	(queue_push_copy): Remove free_src handling.
	(GOMP_OFFLOAD_dev2dev): Update call to queue_push_copy.
	(GOMP_OFFLOAD_openacc_async_host2dev): Remove source-data
	snapshotting.
	(GOMP_OFFLOAD_openacc_async_dev2host): Update call to
	queue_push_copy.
	* target.c (goacc_device_copy_async): Add SRCADDR_ORIG parameter.
	(gomp_copy_host2dev): Add EPHEMERAL parameter.  Snapshot source
	data when true, and set up deferred freeing of temporary buffer.
	(gomp_copy_dev2host): Update call to goacc_device_copy_async.
	(gomp_map_vars_existing, gomp_map_pointer, gomp_attach_pointer)
	(gomp_detach_pointer, gomp_map_vars_internal, gomp_update): Update
	calls to gomp_copy_host2dev with appropriate ephemeral argument.
	* testsuite/libgomp.oacc-c-c++-common/async-data-1-1.c: Remove
	XFAIL.

Co-Authored-By: Thomas Schwinge <thomas@codesourcery.com>

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