c++: bogus warning w/ deduction guide in anon ns [PR106604]

Here we're unintentionally issuing a "declared static but never defined"
warning from wrapup_namespace_globals for a deduction guide declared in
an anonymous namespace.  This patch fixes this by giving deduction guides
a dummy DECL_INITIAL, which suppresses the warning and also allows us to
simplify redeclaration checking for them.

Co-authored-by: Jason Merrill <jason@redhat.com>

	PR c++/106604

gcc/cp/ChangeLog:

	* decl.cc (redeclaration_error_message): Remove special handling
	for deduction guides.
	(grokfndecl): Give deduction guides a dummy DECL_INITIAL.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction74.C: Expect "defined" instead
	of "declared" in the repeated deduction guide diagnostics.
	* g++.dg/cpp1z/class-deduction116.C: New test.