[clang] [-Wunsafe-buffer-usage] Fix a bug and suppress libc warnings for C files (PR #109496)

[Henrik G. Olsson via cfe-commits]

https://github.com/hnrklssn commented:

You may want to add some tests with annotated pointers to verify that you treat them correctly, e.g. `_Nullable` 

https://github.com/llvm/llvm-project/pull/109496