Attached patch is a workaround for the bug. It adds special handling
of lambdas to `cl--sm-macroexpand', similar to the way it handles
`let'. However, there is an unhandled case of "binding" symbols to
forms. I'm also not sure what would be correct behavior in old
non-lexical-scope code (do we still care?).

So, this patch cannot really go in as-is, but it can serve as a base
for a real fix.