[llvm] [UpdateTestChecks] Don't fail silently when conflicting CHECK lines means no checks are generated for some functions (PR #159321)

[Alex Bradbury via llvm-commits]

asb wrote:

Thank you @arichardson for the quick review. I'll land this now (and use this as an opportunity to try out the "auto-merge" button!).

https://github.com/llvm/llvm-project/pull/159321