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

[Erich Keane via cfe-commits]

erichkeane wrote:

> 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.

I agree with all you've said in the review.

Quite a few of the functions listed state that it is UB if they get passed 'nullptr' for the format string, but `printf`/`fprintf` don't do so explicitly (I only spot-checked).  However, the entire standard section for `fprintf` (`printf` says "does what `fprintf` does, to `stdout`") seems to assume a valid format string, so I think this is reasonable.

I'm happy when you are.

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