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

[Nikolas Klauser via cfe-commits]

https://github.com/philnik777 commented:

Is this a good idea? This doesn't just add diagnostics, but also makes it UB to pass a nullptr to these functions. What do actual libc implementations do when they are passed a nullptr? Do any of them define that behaviour?

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