Add Steal::risky_hack_borrow_mut

[Lysxia]

I'm working on a rustc driver (Creusot) which needs to modify the MIR read by two queries, mir_borrowck and check_liveness, in different ways for each query. Both of these queries use mir_promoted to read the MIR, which is immutable (until it is stolen).

This adds an escape hatch so rustc drivers can mutate MIR for specific queries. And this removes get_mut which is unused and also unusable now that there's no way to get a &mut Steal from the rustc API.

Another approach may be to override the queries to modify the MIR after having read it from mir_promoted. However the implementation of queries is largely hidden, so I can't just copy their code to then modify it. A solution would be to parameterize the queries with callbacks which get instantiated with mir_promoted by default, but that seems more involved and ad hoc. That's why I'm proposing this smaller change instead.

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[bjorn3]

I'm pretty sure this will break incr comp badly if used. Please at least give it a much scarier method name.

For modifying mir_promoted a better option may be to instead override the mir_promoted query and within this override call the original query, copy the MIR out of the Steal, modify it and put it back into a new Steal. You can call the original mir_promoted query by doing something like https://github.com/rust-lang/miri/blob/3ab6735110fb308177900c33ff865b5a767979a8/src/bin/miri.rs#L132-L134

[Lysxia]

That's what we were doing before. The problem now is that I want different callers of mir_promoted to get different MIR, to hide different bits from mir_borrowck and from check_liveness.

[Lysxia]

Following your suggestion, I renamed it to something more dissuasive. I am of course open to discuss and implement more robust alternatives!