c++: Fix ICE with implicit operator== [PR94462]

c++: Fix ICE with implicit operator== [PR94462]

duplicate_decls assumed that any TREE_ARTIFICIAL function at namespace scope
was a built-in function, but now in C++20 it's possible to have an
implicitly declared hidden friend operator==.  We just need to move the
assert into the if condition.

gcc/cp/ChangeLog
2020-04-06  Jason Merrill  <jason@redhat.com>

	PR c++/94462
	* decl.c (duplicate_decls): Fix handling of DECL_HIDDEN_FRIEND_P.