c++: Fix use-after-free of replaced friend instantiation [PR118807]

c++: Fix use-after-free of replaced friend instantiation [PR118807]

When instantiating a friend function, we call register_specialization
which adds it to the DECL_TEMPLATE_INSTANTIATIONS of the template.
However, in some circumstances we might immediately call pushdecl and
find an existing specialisation.  In this case, when reregistering the
specialisation we also need to update the DECL_TEMPLATE_INSTANTIATIONS
list so that we don't try to access the freed spec again later.

	PR c++/118807

gcc/cp/ChangeLog:

	* pt.cc (reregister_specialization): Remove spec from
	DECL_TEMPLATE_INSTANTIATIONS.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr118807.C: New test.

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