[clang] [clang][sema] Add nonnull attribute to builtin format functions (PR #158626)

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Sirraide wrote:

> What do actual libc implementations do when they are passed a nullptr? Do any of them define their behaviour?

Glibc at least checks for null and returns `-1`; I haven’t been able to check other libc implementations. I also tried looking at the C standard, but from what I can tell it doesn’t talk about this situation at all, though maybe I missed something

https://github.com/llvm/llvm-project/pull/158626