c++: Adjust module initializer calling emission

We special-case emitting the calls of module initializer functions.  It's
simpler to just emit a static fn do do that, and add it onto the front of
the global init fn chain.  We can also move the calculation of the set of
initializers to call to the point of use.

	gcc/cp/
	* cp-tree.h (module_has_import_init): Rename to ...
	(module_determined_import_inits): ... here.
	* decl2.cc (start_objects): Do not handle module initializers
	here.
	(c_parse_final_cleanups): Generate a separate module
	initializer calling function and add it to the list.  Shrink
	the c-lang region.
	* module.cc (num_init_calls_needed): Delete.
	 (module_has_import_init): Rename to ...
	(module_determined_import_inits): ... here. Do the
	calculation here ...
	(finish_module_processing): ... rather than here.
	(module_add_import_initializers): Reformat.

	gcc/testsuite/
	* g++.dg/modules/init-3_a.C: New.
	* g++.dg/modules/init-3_b.C: New.
	* g++.dg/modules/init-3_c.C: New.