c++: Propagate attributes to clones in duplicate_decls [PR67453]

c++: Propagate attributes to clones in duplicate_decls [PR67453]

On the following testcase where the cdtor attributes aren't on the
in-class declaration but on an out-of-class definition, the cdtors
have their clones created from the in-class declaration, and later on
duplicate_decls updates attributes on the abstract cdtors, but nothing
propagates them to the clones.

2020-11-06  Jakub Jelinek  <jakub@redhat.com>

	PR c++/67453
	* decl.c (duplicate_decls): Propagate DECL_ATTRIBUTES and
	DECL_PRESERVE_P from olddecl to its clones if any.

	* g++.dg/ext/attr-used-2.C: New test.