openmp: Implicitly add 'declare target' directives for dynamic initializers in C++

2020-12-18  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/
	* langhooks-def.h (lhd_get_decl_init): New.
	(lhd_finish_decl_inits): New.
	(LANG_HOOKS_GET_DECL_INIT): New.
	(LANG_HOOKS_OMP_FINISH_DECL_INITS): New.
	(LANG_HOOKS_DECLS): Add LANG_HOOKS_GET_DECL_INIT and
	LANG_HOOKS_OMP_FINISH_DECL_INITS.
	* langhooks.c (lhd_omp_get_decl_init): New.
	(lhd_omp_finish_decl_inits): New.
	* langhooks.h (struct lang_hooks_for_decls): Add omp_get_decl_init
	and omp_finish_decl_inits.
	* omp-offload.c (omp_discover_declare_target_var_r): Use
	get_decl_init langhook in place of DECL_INITIAL.  Call
	omp_finish_decl_inits langhook at end of function.

	gcc/cp/
	* cp-lang.c (cxx_get_decl_init): New.
	(cxx_omp_finish_decl_inits): New.
	(LANG_HOOKS_GET_DECL_INIT): New.
	(LANG_HOOKS_OMP_FINISH_DECL_INITS): New.
	* cp-tree.h (dynamic_initializers): New.
	* decl.c (dynamic_initializers): New.
	* decl2.c (c_parse_final_cleanups): Add initializer entries
	from vars to dynamic_initializers.

	gcc/testsuite/
	* g++.dg/gomp/declare-target-3.C: New.