[clang] [-Wunsafe-buffer-usage] Fix assert when constexpr size passed to snprintf() (#119786) (PR #124022)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

When possible we should always add tests, I think the right place would be:

https://github.com/llvm/llvm-project/blob/main/clang/test/SemaCXX/warn-unsafe-buffer-usage-libc-functions.cpp

CC @ziqingluo-90 

Do we need a release note or is this feature clang-20?

https://github.com/llvm/llvm-project/pull/124022