Implement super let #20058
Description
https://blog.m-ou.se/super-let/
We have implemented rustc's new format_args! expansion but since we are lacking of super let, it makes some borrowck failures like in here
Edit: I initially thought this was causing MIR evaluation errors, but that turned out not to be the case—those were resolved by #20073.
While I believe we'll eventually need this change, it's not urgent at the moment due to other quirks in MIR or borrowck and more pressing issues elsewhere.
So I'm releasing my assignment on this for now.