c++/modules: Support decloned cdtors

c++/modules: Support decloned cdtors

When compiling with '-fdeclone-ctor-dtor' (enabled by default with -Os),
we run into issues where we don't correctly emit the underlying
functions.  We also need to ensure that COMDAT constructors are marked
as such before 'maybe_clone_body' attempts to propagate COMDAT groups to
the new thunks.

gcc/cp/ChangeLog:

	* module.cc (post_load_processing): Mark COMDAT as needed, emit
	declarations if maybe_clone_body fails.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/clone-2_a.C: New test.
	* g++.dg/modules/clone-2_b.C: New test.
	* g++.dg/modules/clone-3_a.C: New test.
	* g++.dg/modules/clone-3_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>