OpenMP: Constructors and destructors for "declare target" static aggregates

This commit also compile-time expands (__builtin_)omp_is_initial_device for
both Fortran and C/C++ (unless, -fno-builtin-omp_is_initial_device is used).
But the main change is:

This commit adds support for running constructors and destructors for
static (file-scope) aggregates for C++ objects which are marked with
"declare target" directives on OpenMP offload targets.

Before this commit, space is allocated on the target for such aggregates,
but nothing ever constructs them properly, so they end up zero-initialised.

(See the new test static-aggr-constructor-destructor-3.C for a reason
why running constructors on the target is preferable to e.g. constructing
on the host and then copying the resulting object to the target.)

2024-08-07  Julian Brown  <julian@codesourcery.com>
	    Tobias Burnus  <tobias@baylibre.com>

gcc/ChangeLog:

	* builtins.def (DEF_GOMP_BUILTIN_COMPILER): Define
	DEF_GOMP_BUILTIN_COMPILER to handle the non-prefix version.
	* gimple-fold.cc (gimple_fold_builtin_omp_is_initial_device): New.
	(gimple_fold_builtin): Call it.
	* omp-builtins.def (BUILT_IN_OMP_IS_INITIAL_DEVICE): Define.
	* tree.cc (get_file_function_name): Support names for on-target
	constructor/destructor functions.

gcc/cp/
	* decl2.cc (tree-inline.h): Include.
	(static_init_fini_fns): Bump to four entries. Update comment.
	(start_objects, start_partial_init_fini_fn): Add 'omp_target'
	parameter. Support "declare target" decls. Update forward declaration.
	(emit_partial_init_fini_fn): Add 'host_fn' parameter. Return tree for
	the created function. Support "declare target".
	(OMP_SSDF_IDENTIFIER): New macro.
	(partition_vars_for_init_fini): Support partitioning "declare target"
	variables also.
	(generate_ctor_or_dtor_function): Add 'omp_target' parameter. Support
	"declare target" decls.
	(c_parse_final_cleanups): Support constructors/destructors on OpenMP
	offload targets.

gcc/fortran/ChangeLog:

	* gfortran.h (gfc_option_t): Add disable_omp_is_initial_device.
	* lang.opt (fbuiltin-): Add.
	* options.cc (gfc_handle_option): Handle
	-fno-builtin-omp_is_initial_device.
	* f95-lang.cc (gfc_init_builtin_functions): Handle
	DEF_GOMP_BUILTIN_COMPILER.
	* trans-decl.cc (gfc_get_extern_function_decl): Add code to use
	DEF_GOMP_BUILTIN_COMPILER for 'omp_is_initial_device'.

libgomp/ChangeLog:

	* testsuite/libgomp.c++/static-aggr-constructor-destructor-1.C: New test.
	* testsuite/libgomp.c++/static-aggr-constructor-destructor-2.C: New test.
	* testsuite/libgomp.c++/static-aggr-constructor-destructor-3.C: New test.
	* testsuite/libgomp.c-c++-common/target-is-initial-host.c: New test.
	* testsuite/libgomp.c-c++-common/target-is-initial-host-2.c: New test.
	* testsuite/libgomp.fortran/target-is-initial-host.f: New test.
	* testsuite/libgomp.fortran/target-is-initial-host.f90: New test.
	* testsuite/libgomp.fortran/target-is-initial-host-2.f90: New test.

Co-authored-by: Tobias Burnus <tobias@baylibre.com>