Auto merge of #33319 - birkenfeld:issue31424, r=jseyfried

borrowck: do not suggest to change "&mut self" to "&mut mut self"

Matching the snippet string might not be the cleanest, but matching
the AST node instead seems to end in a lot of nested `if let`s, so I
don't know what's better.

Of course it's entirely possible that there is another API altogether
that I just don't know of?

Fixes #31424.