[llvm] [Support] report_fatal_error: Do not generate crash backtrace by default (PR #128495)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Changing the default is not the right option as it affects the majority of cases where we shouldn't crash and report proper errors when needed (as library should).

> report_fatal_error, in practice, gets used for things that should be library-safe diagnostics, but aren't "worth it"/we haven't done the work to get it up to the platonic ideal.

These are argument sagainst using report_fatal_error and not about the behavior of report_fatal_error. The current behavior forces tests to run slower, requires adding the extra noise of a --crash argument in tests, and causes confusion by looking like an assertion. 

> If it's only reachable via an invalid state - assert.

Reiterating that there is a spectrum of invalid states. There are cases where it's valid in a low level sense, but should never happen in the system as a whole 

> This I don't follow - MIR is user-facing (as a user of an LLVM release, I can write MIR into a file, then feed it into a tool provided by the LLVM release) 

MIR is 100% not a user facing feature. It is only of use to LLVM backend developers

https://github.com/llvm/llvm-project/pull/128495