c++: friend template matching [PR107484]

Here friend matching tries to find a matching non-template friend and fails,
so we mark the friend as a template specialization to be determined later.
Then cplus_decl_attributes tries again to find a matching function and gets
confused by DECL_TEMPLATE_INSTANTIATION without DECL_TEMPLATE_INFO.  But it
doesn't make sense for find_last_decl to be trying to match anything with
DECL_USE_TEMPLATE set; those are matched elsewhere.

	PR c++/107484

gcc/cp/ChangeLog:

	* decl2.cc (find_last_decl): Return early if DECL_USE_TEMPLATE.

gcc/testsuite/ChangeLog:

	* g++.dg/lookup/friend25.C: New test.