gccrs: ast: Add LangItemPath class

gccrs: ast: Add LangItemPath class

This commit adds a new kind of Path, changes the base Path class and turns TypePath
into a child of the base Path class.

gcc/rust/ChangeLog:

	* ast/rust-path.h (class LangItemPath): New.
	(class TypePath): Adapt to accomodate LangItemPath.
	* ast/rust-ast.cc (TraitImpl::as_string): Use new checks for lang items.
	(QualifiedPathType::as_string): Likewise.
	(FormatArgs::set_outer_attrs): Likewise.
	* ast/rust-item.h (class TraitImpl): Likewise.