Add C++ attribute abi_tag and -Wabi-tag option.

Add C++ attribute abi_tag and -Wabi-tag option.

gcc/
	* attribs.c (lookup_attribute_spec): Handle getting a TREE_LIST.
gcc/c-family/
	* c.opt (Wabi-tag): New.
gcc/cp/
	* tree.c (cxx_attribute_table): Add abi_tag attribute.
	(check_abi_tag_redeclaration, handle_abi_tag_attribute): New.
	* class.c (find_abi_tags_r, check_abi_tags): New.
	(check_bases, check_field_decl): Call check_abi_tags.
	* decl.c (redeclaration_error_message): Call
	check_abi_tag_redeclaration.
	* mangle.c (tree_string_cmp, write_abi_tags): New.
	(write_unqualified_name): Call write_abi_tags.
include/
	* demangle.h (enum demangle_component_type): Add
	DEMANGLE_COMPONENT_TAGGED_NAME.
libiberty/
	* cp-demangle.c (d_dump): Handle DEMANGLE_COMPONENT_TAGGED_NAME.
	(d_make_comp, d_find_pack, d_print_comp): Likewise.
	(d_abi_tags): New.
	(d_name): Call it.

From-SVN: r193367