c++: Fix mismatch in template argument deduction [PR90505]

2020-03-03  Jason Merrill  <jason@redhat.com>
	    Marek Polacek  <polacek@redhat.com>

	PR c++/90505 - mismatch in template argument deduction.
	* pt.c (tsubst): Don't reduce the template level of template
	parameters when tf_partial.

	* g++.dg/template/deduce4.C: New test.
	* g++.dg/template/deduce5.C: New test.
	* g++.dg/template/deduce6.C: New test.
	* g++.dg/template/deduce7.C: New test.