c++: Fix OpenMP support with C++20 modules [PR119864]

c++: Fix OpenMP support with C++20 modules [PR119864]

In r15-2799-gf1bfba3a9b3f31, a new kind of global constructor was added.
Unfortunately this broke C++20 modules, as both the host and target
constructors were given the same mangled name.  This patch ensures that
only the host constructor gets the module name mangling for now, and
stops forcing the creation of the target constructor even when no such
initialization is required.

	PR c++/119864

gcc/cp/ChangeLog:

	* decl2.cc (start_objects): Only use module initialized for
	host.
	(c_parse_final_cleanups): Don't always create an OMP offload
	init function in modules.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/openmp-1.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>