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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@43156 91177308-0d34-0410-b5e6-96231b3b80d8