[llvm] [OrcJIT] distinguish normal and abrupt TaskDispatcher shutdown (PR #154356)

[Jameson Nash via llvm-commits]

vtjnash wrote:

Would you be willing to merge this as essentially a progress meter towards that goal? I didn't want to yet complicate the tests by converting them, so I had intended those parts of the changes to merely help identify which tests could be improved with promise/future at a later date, and to unblock other work on testing better schedulers.

Aside from that, the concept of run-to-complete is usually quite important for correctness in my applications, and I already maintain several runtimes that provide this for other uses so it isn't novel to me (notably julia and libuv/nodejs). For clarification also, this function is simply exposing the way that worker threads creation is implemented and giving it a name.

https://github.com/llvm/llvm-project/pull/154356