c++: Fix tsubsting member variable template-id [PR96330]

This makes tsubst_copy appropriately handle a variable template-id, which
in turn fixes tsubsting a COMPONENT_REF whose member operand is known at
parse time to be a variable template-id, as in the initialization of 'x'
in the first testcase.  Previously, we rejected this testcase with the
error "foo_t::bar<T> is not a function template", issued from
lookup_template_fuction.

We were already properly handling the analagous case where the object
operand of the COMPONENT_REF is dependent (and so the member operand is
a dependent template name), but there doesn't seems to be existing test
coverage for this, hence the second testcase below.

gcc/cp/ChangeLog:

	PR c++/96330
	* pt.c (tsubst_copy) <case TEMPLATE_ID_EXPR>: Rename local
	variable 'fn' to 'tmpl'.  Handle a variable template-id by
	calling lookup_template_variable.

gcc/testsuite/ChangeLog:

	PR c++/96330
	* g++.dg/cpp1y/var-templ68.C: New test.
	* g++.dg/cpp1y/var-templ68a.C: New test.

Co-authored-by: Jakub Jelinek <jakub@redhat.com>

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