c++: non-dependent variable template-id [PR108848]

Here we're treating deeming the non-dependent variable template-id
tag<int> as dependent ever since r226642 gave variable TEMPLATE_ID_EXPR
an empty type, which causes the call to finish_template_variable from
finish_id_expression_1 to be unreachable at template parse time.  Thus
we're led into thinking tag<int>.var<void> refers to a dependent name.

This patch fixes this by making finish_id_expression_1 instantiate a
variable template-id as long as it's not dependent according to the
dependence test in lookup_and_finish_template_variable rather than
according to type_dependent_expression_p.

	PR c++/108848

gcc/cp/ChangeLog:

	* pt.cc (finish_template_variable): Move dependence check
	to here from ...
	(lookup_and_finish_template_variable): ... here.
	* semantics.cc (finish_id_expression_1): Call
	finish_template_variable sooner, before (and regardless of) the
	type_dependent_expression_p test.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/noexcept1.C: Don't expect a bogus "different
	exception specifier" error.  Expect a separate "not usable
	in a constant expression" error.
	* g++.dg/cpp1y/var-templ75.C: New test.
	* g++.dg/cpp1y/var-templ76.C: New test.