[libc-commits] [libc] [libc][NFC] Correct test header inclusion, license (PR #114604)

[Nick Desaulniers via libc-commits]

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

A common pattern to keep this from creeping back in is to have the header we expect users to include #define something, then in the header we don't want users to include check for that define and error with a helpful message that they should instead include the parent header.

https://github.com/llvm/llvm-project/pull/114604