c++: over-eager friend matching [PR109649]

A bug in the simplification I did around 91618; at this point X<int>::f has
DECL_IMPLICIT_INSTANTIATION set, but we've already identified what template
it corresponds to, so we don't want to call check_explicit_specialization.

To distinguish this case we need to look at DECL_TI_TEMPLATE.  grokfndecl
has for a long time set it to the OVERLOAD in this case, while the new cases
I added for 91618 were leaving DECL_TEMPLATE_INFO null; let's adjust them to
match.

	PR c++/91618
	PR c++/109649

gcc/cp/ChangeLog:

	* friend.cc (do_friend): Don't call check_explicit_specialization if
	DECL_TEMPLATE_INFO is already set.
	* decl2.cc (check_classfn): Set DECL_TEMPLATE_INFO.
	* name-lookup.cc (set_decl_namespace): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/template/friend77.C: New test.