debuginfo: Fix regression in for-loop variable debuginfo ...

... really this time `:)`

I went for the simpler fix after all since it turned out to become a bit too complicated to extract the current iteration value from its containing `Option` with the different memory layouts it can have. It's also what we already do for match bindings.

I also extended the new test case to include the "simple identifier" case.

Fixes #20127, fixes #19732