PR c++/42317
	* cgraph.h (struct cgraph_node): Add same_comdat_group field.
	* cgraph.c (cgraph_remove_node): Unchain node from same_comdat_group
	circular list.
	(cgraph_node_can_be_local_p): Return false for DECL_COMDAT with
	node->same_comdat_group.
	* ipa.c (cgraph_remove_unreachable_nodes): For any reachable node
	mark all its same_comdat_group nodes as also reachable.
	(cgraph_externally_visible_p): Return true even if any of
	same_comdat_group nodes has address taken.
	* lto-cgraph.c (lto_output_node): Stream out same_comdat_group.
	(output_cgraph): Ensure other same_comdat_group nodes are also
	included.
	(input_node): Stream in same_comdat_group.
	(input_cgraph_1): Fix up same_comdat_group fields from references
	to pointers.
	* cgraphunit.c (cgraph_analyze_functions): Mark all other
	same_comdat_group nodes as reachable.
	(cgraph_mark_functions_to_output): For each node->process process
	also other same_comdat_group nodes.
	* ipa-inline.c (cgraph_clone_inlined_nodes): Don't reuse nodes
	with same_comdat_group non-NULL.
	(cgraph_mark_inline_edge): Likewise.

	* decl2.c (cp_write_global_declarations): Clear DECL_EXTERNAL
	also on all other functions in the same comdat group.
	* optimize.c (maybe_clone_body): Also optimize virtual implicit
	dtors.  For virtual comdat dtors tell cgraph that base and deleting
	dtor are in the same comdat group.

	* config/abi/pre/gnu.ver: Don't export certain base dtors that
	weren't previously exported.

	* g++.dg/opt/dtor2.C: New test.
	* g++.dg/opt/dtor2.h: New file.
	* g++.dg/opt/dtor2-aux.cc: New file.

From-SVN: r155143