Fixed DeclStmt::child_begin() to actually create an iterator that

visits its decls, rather than just creating an "end()" iterator.

Fixed child_end() for statements and expressions to use
child_iterator() to create the end() iterator, rather than just
returning NULL.

Fixed bug in StmtIterator where we did not correctly detect if we had
marched off the end of the ScopedDecls.

llvm-svn: 43156