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

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https://github.com/Sirraide commented:

I’m not sure it’s defined anywhere whether `nullptr` is a valid argument to `printf`, but if GCC diagnoses this then it should be fine for us to do so as well.

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