NLL: incorrect "unused mut" lint warning, with multiple closures

[lqd]

Found while trying to bootstrap Rust with NLL enabled, just in case we're not already tracking this incorrect warning (as we're already tracking other similar ones in #51801 and #51830 and more).

Reduced from libsyntax::parser::parser::parse_dot_or_call_expr_with. Probably not exactly minimal, but smaller than the whole parser :)

Repro here on the playground

[nikomatsakis]

Tagging as NLL-deferred because #51918 has precedence