c++: friend implicit template instantiation [PR91618]

c++: friend implicit template instantiation [PR91618]

This rule that for a friend with a qualified name we try to find a
matching template was already in C++98, but it seems we never implemented
it, and nobody reported it until 2019.

This patch sets DECL_IMPLICIT_INSTANTIATION to signal to
check_explicit_specialization that we want to find a template, like
grokfndecl already did for explicit template args.  check_classfn also needs
to call it, as check_classfn is called after the call to
check_explicit_specialization in grokfndecl, whereas the call to
set_decl_namespace comes sooner.  This inconsistency is inelegant, but safer
at this point in the release cycle; I'll unify them in stage 1.

	PR c++/91618
	PR c++/96604

gcc/cp/ChangeLog:

	* name-lookup.cc (set_decl_namespace): Set
	DECL_IMPLICIT_INSTANTIATION if no non-template match.
	* pt.cc (check_explicit_specialization): Check it.
	* decl2.cc (check_classfn): Call it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/friend7.C: Remove xfail.
	* g++.dg/template/friend72.C: New test.
	* g++.dg/template/friend72a.C: New test.
	* g++.dg/template/friend73.C: New test.