c++/modules: Fix handling of -fdeclone-ctor-dtor with explicit instantiations [PR120125]

c++/modules: Fix handling of -fdeclone-ctor-dtor with explicit instantiations [PR120125]

The attached testcase ICEs in maybe_thunk_body because we haven't
created a node in the cgraph for an imported explicit instantiation yet.

We in fact really shouldn't be emitting calls at all, since an imported
explicit instantiation always exists in the TU we imported it from.  But
the required logic for that doesn't exist in this branch, so we'll just
xfail the relevant check.

	PR c++/120125

gcc/cp/ChangeLog:

	* optimize.cc (maybe_thunk_body): Don't assume 'fn' has a cgraph
	node created.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/clone-4_a.C: New test.
	* g++.dg/modules/clone-4_b.C: New test.

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