Unify storage of associated items in SingleASTNode

Unify storage of associated items in SingleASTNode

gcc/rust/ChangeLog:

	* ast/rust-ast-fragment.cc
	(Fragment::assert_single_fragment): Update.

	* ast/rust-ast.h
	(class TraitImplItem): Move definition before that of TraitItem.
	(class TraitItem):
	Inherit from TraitImplItem instead of AssociatedItem.

	(class SingleASTNode): Unify handling of associated items.
	(SingleASTNode::take_assoc_item): Move from...
	(SingleASTNode::take_impl_item): ...here, but leave stub calling
	take_assoc_item behind.
	(SingleASTNode::take_trait_item):
	Cast associated item to TraitItem.
	(SingleASTNode::take_trait_impl_item):
	Cast associated item to TraitImplItem.

	* ast/rust-ast.cc
	(SingleASTNode::SingleASTNode):
	Unify handling of associated items.
	(SingleASTNode::operator=): Likewise.
	(SingleASTNode::accept_vis): Likewise.
	(SingleASTNode::is_error): Likewise.
	(SingleASTNode::as_string): Likewise.

	* ast/rust-item.h
	(class Function): Remove direct inheritence from AssociatedItem.
	(class ConstantItem): Likewise.
	* ast/rust-macro.h
	(class MacroInvocation):
	Remove direct inheritence from AssociatedItem and TraitImplItem.