[clang] [-Wunsafe-buffer-usage] Add check for custom printf/scanf functions (PR #173096)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> Sadly, our CI reports an assertion failing when building the re2c library (version 3.1):

Are you able to provide a stack trace, and if possible the repro case that should have been produced on the crash?

https://github.com/llvm/llvm-project/pull/173096