[llvm] [LLVM] Parametrize hardcoded behaviors in diagnostics error handling. (PR #156439)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

I do not like that today we have a difference in error behavior between llc and opt. This PR almost helps with that, but it adds yet another mechanism parallel to the existing diagnostic callback. Can we just make it not an option, and always do what llc does?

In particular, I think the behavior of exiting is always wrong. llc has the correct and more useful behavior for writing error tests. 

https://github.com/llvm/llvm-project/pull/156439