c++: Don't ICE in eval_constant_of on vars with error_mark_node type [PR125412]

The following testcase ICEs during error recovery, because we aren't
prepared to handle error_mark_node type.

Fixed thusly.

2026-05-27  Jakub Jelinek  <jakub@redhat.com>

	PR c++/125412
	* reflect.cc (process_metafunction): If ht is error_operand_p,
	set *non_constant_p and return NULL_TREE.

	* g++.dg/reflect/pr125412.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 1124bb120dea57071537465033b38da914fc16da)