Switch to begin_panic again

[est31]

In #42938 we made the compiler
emit a call to begin_panic_new in order to pass column info to it. Now
with stage0 updated (#43320),
we can safely change begin_panic and start emitting calls for it again.

[rust-highfive]

r? @alexcrichton

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[est31]

I'm not sure whether I could have spared one stage0 update (I have chosen a strategy to update the abi over two stage0 updates) by making a #[cfg(stage0)] version of begin_panic with the old abi and a #[not(cfg(stage0))] version with the new abi. It might have caused issues with proc macros, not sure. In any case, now is too late to switch strategies, we need to wait for a second stage0 update to get rid of the #[cfg(stage0)] stuff that this PR adds.

[alexcrichton]

@bors: r+

[bors]

📌 Commit 90ac640 has been approved by alexcrichton

[bors]

⌛ Testing commit 90ac640 with merge e574ba4...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing e574ba4 to master...