c++: Avoid -Wreturn-type warning if a template fn calls noreturn template fn [PR94742]

finish_call_expr already has code to set current_function_returns_abnormally
if a template calls a noreturn function, but on the following testcase it
doesn't call a FUNCTION_DECL, but TEMPLATE_DECL instead, in which case
we didn't check noreturn at all and just assumed it could return.

2020-04-25  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94742
	* semantics.c (finish_call_expr): When looking if all overloads
	are noreturn, use STRIP_TEMPLATE to look through TEMPLATE_DECLs.

	* g++.dg/warn/Wreturn-type-12.C: New test.