[PATCH] D79507: [flang] Change DIE("unreachable") cases to use llvm_unreachable

[David Truby via Phabricator via llvm-commits]

DavidTruby added a subscriber: mehdi_amini.
DavidTruby added a comment.

> That all having been said, I'm open to having more granular flags for essentially selecting NDEBUG on a subproject level when configuring. I can certainly see why that might be convenient while working on development of a particular subproject for running test suites and the like.

I'm fine with this, it's something that @mehdi_amini and I both proposed as a way forward in a previous discussion on this: (see here https://github.com/flang-compiler/f18/issues/966). It's also not an issue building flang without NDEBUG and llvm with; we had accidentally been doing this since merging until I fixed the discrepancy yesterday (see here https://reviews.llvm.org/rG81bf1e29aae5992db318803ba4722a585ad64638). 
I would want this toggleable flag to still be toggling the standard LLVM error handling mechanisms though, namely `llvm_unreachable` for unreachable cases and `assert` for function preconditions, so that people submitting code for flang can follow the LLVM coding guidelines and not have their patches rejected on those grounds.


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