c++: ICE with loopy var tmpl auto deduction [PR109300]

Now that we resolve non-dependent variable template-ids ahead of time,
cp_finish_decl needs to handle a new invalid situation: we can end up
trying to instantiate a variable template with deduced type before we
fully parsed its initializer.

	PR c++/109300

gcc/cp/ChangeLog:

	* decl.cc (cp_finish_decl): Diagnose ordinary auto deduction
	with no initializer, instead of asserting.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/var-templ79.C: New test.