Support tail calls in mir via TerminatorKind::TailCall

[WaffleLapkin]

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

This adds a new TerminatorKind which represents a tail call:

    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.

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

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r? @oli-obk
cc @scottmcm @drmeepster @JakobDegen