Implement P0840, language support for empty objects.

Implement P0840, language support for empty objects.

	The [[no_unique_address]] attribute on a non-static data member
	enables the equivalent of the empty base optimization.

gcc/cp/
	* tree.c (handle_no_unique_addr_attribute): New.
	(cxx_attribute_table): Add [[no_unique_address]].
	* class.c (field_poverlapping_p): New.
	(layout_class_type): Check it.  Adjust DECL_SIZE of potentially
	overlapping fields.
	(layout_empty_base_or_field): Rename from layout_empty_base, handle
	FIELD_DECL as well.
	(build_base_field, record_subobject_offsets): Adjust.
c-family/
	* c-lex.c (c_common_has_attribute): Add no_unique_address.

From-SVN: r264813