Clone .used upvars eagerly in non-move closures

[Jules-Bertholet]

By not keeping a reference to the capture, we extend the closure's lifetime.

This reverts #157270.

cc @joshtriplett, as the author of rust-lang/rfcs#3680

@rustbot label A-closures F-ergonomic_clones

Tracking issue: #132290

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[hanna-kruppe]

I'm not familiar with this lang experiment and don't know its current status or future prospects. People also expressed uncertainty about that in #157270. I can just re-roll (wouldn't be very qualified to review this anyway) but I wonder if we're beating a dead horse feature?

[Jules-Bertholet]

r? @joshtriplett

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