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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> it is meant to crash on internal bug. 

No, it is not. It is a clean error on compiler developer bug, or area where a proper diagnostic isn't worth the effort.

> report_fatal_error is not meant to handle invalid inputs to the compiler:

There are a range of cases where the meaning of "invalid" and "input to the compiler" are not clear. abort-with-backtrace is not appropriate. 

> Compiler ICE isn't correct error handling.

It is in some scenarios. There is not a hard line between distinguishing these cases. There is a chain of error quality degradation. report_fatal_error is one step above assert, but should be a clean error. For cases which are never supposed to be user facing, but are compiler developer facing, report_fatal_error is the correct tool. In particular, the invalid MIR input case. I hand write MIR all day and expect to see a clean error. There are also cases which cannot or should not be promoted to IR verifier error, and still need some kind of minimal effort to avoid a crash in the backend. 

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