c++: Fix deduction with reference NTTP [PR83476]

c++: Fix deduction with reference NTTP [PR83476]

In the testcase ref11.C below, during deduction for the call f(a),
uses_deducible_template_parms returns false for the dependent
specialization A<V> because the generic template argument V here is
wrapped in an implicit INDIRECT_REF (formed from template_parm_to_arg).
Since uses_deducible_template_parms returns false, unify_one_argument
exits early without ever attempting to deduce 'n' for 'V'.  This patch
fixes this by making deducible_expression look through such implicit
INDIRECT_REFs.

gcc/cp/ChangeLog:

	PR c++/83476
	PR c++/99885
	* pt.c (deducible_expression): Look through implicit
	INDIRECT_REFs as well.

gcc/testsuite/ChangeLog:

	PR c++/83476
	PR c++/99885
	* g++.dg/cpp1z/class-deduction85.C: New test.
	* g++.dg/template/ref11.C: New test.

(cherry picked from commit 2ccc05a5141506fde0e20dec702c717fd67bf6ee)