c++: abbreviated function template friend matching [PR96106]

In the below testcase, duplicate_decls wasn't merging the tsubsted
friend declaration for 'void add(auto)' with its definition, because
reduce_template_parm_level (during tsubst_friend_function) lost the
DECL_VIRTUAL_P flag on the auto's invented template parameter, which
caused template_heads_equivalent_p to deem the two template heads as not
equivalent in C++20 mode.

This patch makes reduce_template_parm_level carry over the
DECL_VIRTUAL_P flag from the original TEMPLATE_PARM_DECL.

gcc/cp/ChangeLog:

	PR c++/96106
	* pt.c (reduce_template_parm_level): Propagate DECL_VIRTUAL_P
	from the original TEMPLATE_PARM_DECL to the new lowered one.

gcc/testsuite/ChangeLog:

	PR c++/96106
	* g++.dg/concepts/abbrev7.C: New test.