[Runtime] Add an SPI to register a callback for thrown errors.

[mikeash]

rdar://problem/53400364

[stmontgomery]

Don't have a strong opinion, but maybe _addWillThrowErrorCallback would be a clearer name? (i.e. "will throw error" instead of "error will throw".) Just since Error types don't perform the throw, they are thrown.

[jckarter]

What are you trying to do? swift_willThrow is not reliably emitted by the compiler, and I don't think we want to lock down the places we happen to emit it today. It gets optimized out of release builds, and gets emitted too often in debug builds (you'll get a callback along every implicit propagation edge, not just at the throw statement). I don't think it's suitable for adding any logic a program might depend on in its current state. Even if it were more reliably emitted, I don't think we'd want to expose this even as SPI, because it's easy to misuse, and we wouldn't be able to prevent people from doing so. It's semantically problematic because the optimizer expects throwing or error propagation to be a pure operation like return; inlining or control flow transformations could arbitrary introduce or remove throws.

[mikeash]

@jckarter and I discussed this some more offline and we decided that a function pointer like _swift_retain would be a decent way to do this. That makes it more obviously dangerous and less prone to weird abuse, and it won't show up in the API (even with a leading underscore).

Since that's fairly different from what I have here, I've created a new PR for that over at #27863.