Incorrect indentation with `let` in guards

```
module Foo where

foo :: Int -> IO ()
foo x = case x of
x |
let y = x + 1,
y > 5 -> do
putStrLn "foo"
```

If you load this file in haskell-mode and then press TAB on the line containing `putStrLn`, it gets dedented all the way to the left, which is incorrect.

Reproduce the issue in haskell-mode by loading the provided Foo module and pressing TAB on the putStrLn line. Inspect the indentation handling for a case guard containing a let binding, and consider the issue done when that line remains nested under the guard and do block.

Written by the indexing model from the issue text.