c++: Don't support auto{x} and auto(x) for C++ < 23 in SFINAE contexts [PR120685]

The following testcase ICEs in C++ 20 and older, because during
diagnostics dump_template_bindings attempts to tsubst with tf_none
something and we try to emit a pedwarn during that.
While the pedwarn could be just guarded with if (complain & tf_warning),
I thought we shouldn't support extensions for tf_none and should
instead return error_mark_node.

The following patch does that.

This is a backport version which instead of not supporting it just
doesn't emit the pedwarn in SFINAE contexts for C++ < 23 but still
supports it.

2026-02-17  Jakub Jelinek  <jakub@redhat.com>

	PR c++/120685
	* typeck2.cc (build_functional_cast_1): For C++23 auto(x)
	without tf_warning or tf_error return error_mark_node instead of
	emitting pedwarn and handling it.
	* semantics.cc (finish_compound_literal): Similarly for C++26
	auto{x}.

	* g++.dg/cpp23/auto-fncast19.C: New test.

(cherry picked from commit f7efb7e62ac96c082ff72549d9b3789f5c955efe)