c++: variable template and targ deduction [PR108550]

In this test, we get a bogus error because we failed to deduce the auto in
constexpr auto is_pointer_v = is_pointer<Tp>::value;
to bool.  Then ensure_literal_type_for_constexpr_object thinks the object
isn't literal and an error is reported.

This is another case of the interaction between tf_partial and 'auto',
where the auto was not reduced so the deduction failed.  In more detail:
we have

  Wrap1<int>()

in the code and we need to perform OR -> fn_type_unification.  The targ
list is incomplete, so we do
      tsubst_flags_t ecomplain = complain | tf_partial | tf_fndecl_type;
      fntype = tsubst (TREE_TYPE (fn), explicit_targs, ecomplain, NULL_TREE);
where TREE_TYPE (fn) is struct integral_constant <T402> (void).  Then
we substitute the return type, which results in tsubsting is_pointer_v<int>.
is_pointer_v is a variable template with a placeholder type:

  template <class Tp>
  constexpr auto is_pointer_v = is_pointer<Tp>::value;

so we find ourselves in lookup_and_finish_template_variable.  tf_partial is
still set, so finish_template_variable -> instantiate_template -> tsubst
won't reduce the level of auto.  But then we do mark_used which eventually
calls do_auto_deduction which clears tf_partial, because we want to replace
the auto now.  But we hadn't reduced auto's level so this fails.  And
since we're not in an immediate context, we emit a hard error.

I suppose that when we reach lookup_and_finish_template_variable it's
probably time to clear tf_partial.  (I added an assert and our testsuite
doesn't have a test whereby we get to lookup_and_finish_template_variable
while tf_partial is still active.)

	PR c++/108550

gcc/cp/ChangeLog:

	* pt.cc (lookup_and_finish_template_variable): Clear tf_partial.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/var-templ70.C: New test.
	* g++.dg/cpp1y/var-templ71.C: New test.
	* g++.dg/cpp1y/var-templ72.C: New test.