ast: Introduce class hierarchy for lang item paths

ast: Introduce class hierarchy for lang item paths

Create a base Path class which is derived into two children classes for
regular paths and lang item paths. This allows it to hold either the
segments of a fully formed path, or the node ID of a lang-item path.
This is required in order to create these special paths
which do not have segments, and do not necessarily have a canonical
form - they only depend on where the item was defined.

gcc/rust/ChangeLog:

	* ast/rust-ast-full-decls.h (class PathPattern): Rename PathPattern to...
	(class Path): ...Path
	* ast/rust-ast-collector.cc (TokenCollector::visit): Add required methods
	for LangItemPath and RegularPath.
	* ast/rust-ast-collector.h: Likewise.
	* ast/rust-ast-visitor.cc (DefaultASTVisitor::visit): Likewise.
	* ast/rust-ast-visitor.h: Likewise.
	* ast/rust-path.cc (PathPattern::as_string): Likewise.
	(RegularPath::as_string): Likewise.
	(LangItemPath::as_string): Likewise.
	(PathPattern::convert_to_simple_path): Likewise.
	(RegularPath::convert_to_simple_path): Likewise.
	(RegularPath::accept_vis): Likewise.
	(LangItemPath::accept_vis): Likewise.
	(PathInExpression::as_string): Likewise.
	(QualifiedPathInExpression::as_string): Likewise.
	* ast/rust-path.h (class PathPattern): Likewise.
	(class Path): Likewise.
	(class RegularPath): Likewise.
	(class LangItemPath): Likewise.
	(class PathInExpression): Likewise.
	(class QualifiedPathInExpression): Likewise.
	* ast/rust-pattern.h (class PathPattern): Likewise.
	(class Path): Likewise.
	* expand/rust-derive.h: Likewise.
	* hir/rust-ast-lower-base.cc (ASTLoweringBase::visit): Likewise.
	* hir/rust-ast-lower-base.h: Likewise.
	* resolve/rust-ast-resolve-base.cc (ResolverBase::visit): Likewise.
	* resolve/rust-ast-resolve-base.h: Likewise.