c++: Better module initializer code

c++: Better module initializer code

Every module interface needs to emit a global initializer, but it
might have nothing to init.  In those cases, there's no need for any
idempotency boolean to be emitted.

	gcc/cp
	* cp-tree.h (module_initializer_kind): Replace with ...
	(module_global_init_needed, module_has_import_inits): ...
	these.
	* decl2.cc (start_objects): Add has_body parm.  Reorganize
	module initializer creation.
	(generate_ctor_or_dtor_function): Adjust.
	(c_parse_final_cleanups): Adjust.
	(vtv_start_verification_constructor_init_function): Adjust.
	* module.cc (module_initializer_kind): Replace with ...
	(module_global_init_needed, module_has_import_inits): ...
	these.

	gcc/testsuite/
	* g++.dg/modules/init-2_a.C: Check no idempotency.
	* g++.dg/modules/init-2_b.C: Check idempotency.