c++: converted lambda as template argument [PR83258, ...]

r8-1253-g3d2e25a240c711 removed the template argument linkage requirement
in convert_nontype_argument for C++17 (which r9-3836-g4be5c72cf3ea3e later
factored out into invalid_tparm_referent_p), but we need to also remove
the one in convert_nontype_argument_function for benefit of the first and
third testcase which we currently reject even in C++17/20 mode.

And in invalid_tparm_referent_p we're inadvertendly returning false for
the address of a lambda's static op() since it's DECL_ARTIFICIAL, which
currently causes us to reject the second (C++20) testcase.  But this
DECL_ARTIFICIAL check seems to be relevant only for VAR_DECL, and in fact
this code path was originally reachable only for VAR_DECL until recently
(r13-6970-gb5e38b1c166357).  So this patch restricts the check to VAR_DECL.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

	PR c++/83258
	PR c++/80488
	PR c++/97700

gcc/cp/ChangeLog:

	* pt.cc (convert_nontype_argument_function): Remove linkage
	requirement for C++17 and later.
	(invalid_tparm_referent_p) <case ADDR_EXPR>: Restrict
	DECL_ARTIFICIAL rejection test to VAR_DECL.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/visibility/anon8.C: Don't expect a "no linkage"
	error for the template argument &B2:fn in C++17 mode.
	* g++.dg/cpp0x/lambda/lambda-conv15.C: New test.
	* g++.dg/cpp2a/nontype-class56.C: New test.
	* g++.dg/template/function2.C: New test.

(cherry picked from commit c3afdb8b)