c++: ICE during aggr CTAD for member tmpl [PR105476]

c++: ICE during aggr CTAD for member tmpl [PR105476]

Here we're crashing from maybe_aggr_guide ultimately because
processing_template_decl isn't set when partially instantiating the
guide's parameter list; this causes us to force completion of the
dependent type Visitor_functior<Fn>, which of course fails and results
in an unexpected error_mark_node (the instantation should always succeed).

	PR c++/105476

gcc/cp/ChangeLog:

	* pt.cc (maybe_aggr_guide): Set processing_template_decl when
	partially instantiating the guide's parameter list.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/class-deduction-aggr13.C: New test.
	* g++.dg/cpp2a/class-deduction-aggr13a.C: New test.

(cherry picked from commit 8a98e3ff7e80bf2936f163d50309fd88d72564a0)