Auto merge of #113128 - WaffleLapkin:become_trully_unuwuable, r=oli-o…

…bk,RalfJung

Support tail calls in mir via `TerminatorKind::TailCall`

This is one of the interesting bits in tail call implementation — MIR support.

This adds a new `TerminatorKind` which represents a tail call:
```rust
    TailCall {
        func: Operand<'tcx>,
        args: Vec<Operand<'tcx>>,
        fn_span: Span,
    },
```

*Structurally* this is very similar to a normal `Call` but is missing a few fields:
- `destination` — tail calls don't write to destination, instead they pass caller's destination to the callee (such that eventual `return` will write to the caller of the function that used tail call)
- `target` — similarly to `destination` tail calls pass the caller's return address to the callee, so there is nothing to do
- `unwind` — I _think_ this is applicable too, although it's a bit confusing
- `call_source` — `become` forbids operators and is not created as a lowering of something else; tail calls always come from HIR (at least for now)

It might be helpful to read the interpreter implementation to understand what `TailCall` means exactly, although I've tried documenting it too.

-----

There are a few `FIXME`-questions still left, ideally we'd be able to answer them during review ':)

-----

r? `@oli-obk`
cc `@scottmcm` `@DrMeepster` `@JakobDegen`