[clang] [clang] Remove "unknown" from availability diags (PR #138610)

[Cyndy Ishida via cfe-commits]

cyndyishida wrote:

> The change looks reasonable but can you add a test? thanks!

Open to other suggestions @cor3ntin, but I didn't figure out a way to ensure part of a diagnostic was omitted with cc1's `-verify` so its a driver test that uses `FIleCheck`

https://github.com/llvm/llvm-project/pull/138610