[libc-commits] [libc] [libc] Keep headers compatible with -std=c89 mode (PR #121981)

[Joseph Huber via libc-commits]

https://github.com/jhuber6 approved this pull request.

Do we need to do this for `__inline__` as well? I forget if that shows up anywhere.

https://github.com/llvm/llvm-project/pull/121981