c++: Named module global initializers

c++: Named module global initializers

C++ 20 modules adds some new rules about when the global initializers
of imported modules run.  They must run no later than before any
initializers in the importer that appear after the import.  To provide
this, each named module emits an idempotent global initializer that
calls the global initializer functions of its imports (these of course
may call further import initializers).  This is the machinery in our
global-init emission to accomplish that, other than the actual
emission of calls, which is in the module file.  The naming of this
global init is a new piece of the ABI.

FWIW, the module's emitter does some optimization to avoid calling a
direct import's initializer when it can determine thatr import is also
indirect.

	gcc/cp/
	* decl2.c (start_objects): Refactor and adjust for named module
	initializers.
	(finish_objects): Likewise.
	(generate_ctor_or_dtor_function): Likewise.
	* module.cc (module_initializer_kind)
	(module_add_import_initializers): Stubs.