tree.c (walk_tree): Expose tail recursion.

	* tree.c (walk_tree): Expose tail recursion.
	(walk_stmt_tree): New function.
	* cp-tree.h: Prototype walk_stmt_tree.
	* semantics.c (prune_unused_decls): Operate on SCOPE_STMTs not
	the BLOCKs directly.  If a BLOCK has no variables after
	pruning, discard it.
	(finish_stmt_tree): Use walk_stmt_tree.  No need to save and
	restore the line number.

From-SVN: r36178