openacc: Adjust dynamic reference count semantics

This patch adjusts how dynamic reference counts work so that they match
the semantics of the source program more closely, instead of representing
"excess" reference counts beyond those that represent pointers in the
internal libgomp splay-tree data structure. This allows some corner
cases to be handled more gracefully.

2020-07-10  Julian Brown  <julian@codesourcery.com>
	    Thomas Schwinge  <thomas@codesourcery.com>

	libgomp/
	* libgomp.h (struct splay_tree_key_s): Change virtual_refcount to
	dynamic_refcount.
	(struct gomp_device_descr): Remove GOMP_MAP_VARS_OPENACC_ENTER_DATA.
	* oacc-mem.c (acc_map_data): Substitute virtual_refcount for
	dynamic_refcount.
	(acc_unmap_data): Update comment.
	(goacc_map_var_existing, goacc_enter_datum): Adjust for
	dynamic_refcount semantics.
	(goacc_exit_datum_1, goacc_exit_datum): Re-add some error checking.
	Adjust for dynamic_refcount semantics.
	(goacc_enter_data_internal): Implement "present" case of dynamic
	memory-map handling here.  Update "non-present" case for
	dynamic_refcount semantics.
	(goacc_exit_data_internal): Use goacc_exit_datum_1.
	* target.c (gomp_map_vars_internal): Remove
	GOMP_MAP_VARS_OPENACC_ENTER_DATA handling.  Update for dynamic_refcount
	handling.
	(gomp_unmap_vars_internal): Remove virtual_refcount handling.
	(gomp_load_image_to_device): Substitute dynamic_refcount for
	virtual_refcount.
	* testsuite/libgomp.oacc-c-c++-common/pr92843-1.c: Remove XFAILs.
	* testsuite/libgomp.oacc-c-c++-common/refcounting-1.c: New test.
	* testsuite/libgomp.oacc-c-c++-common/refcounting-2.c: New test.
	* testsuite/libgomp.oacc-c-c++-common/struct-3-1-1.c: New test.
	* testsuite/libgomp.oacc-fortran/deep-copy-6.f90: Remove XFAILs and
	trace output.
	* testsuite/libgomp.oacc-fortran/deep-copy-6-no_finalize.F90: Remove
	trace output.
	* testsuite/libgomp.oacc-fortran/dynamic-incr-structural-1.f90: New
	test.
	* testsuite/libgomp.oacc-c-c++-common/structured-dynamic-lifetimes-4.c:
	Remove stale comment.
	* testsuite/libgomp.oacc-fortran/mdc-refcount-1-1-1.f90: Remove XFAILs.
	* testsuite/libgomp.oacc-fortran/mdc-refcount-1-1-2.F90: Likewise.
	* testsuite/libgomp.oacc-fortran/mdc-refcount-1-2-1.f90: Likewise.
	* testsuite/libgomp.oacc-fortran/mdc-refcount-1-2-2.f90: Likewise.
	* testsuite/libgomp.oacc-fortran/mdc-refcount-1-3-1.f90: Likewise.
	* testsuite/libgomp.oacc-fortran/mdc-refcount-1-4-1.f90: Adjust XFAIL.

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