let with no bindings in do notation causes parse error

Originally from https://github.com/ndmitchell/hlint/issues/368 by @bucklereed, taking:

```
foo = do
  let
  Just 3

bar :: ()
bar = ()
```

If I run with HSE:

    Prelude Language.Haskell.Exts> parse "foo = do\n let\n Just 3\n\nbar :: ()\nbar = ()" :: ParseResult (Module SrcSpanInfo)
    ParseFailed (SrcLoc "<unknown>.hs" 5 1) "Parse error: bar"

This works with GHC.