[clang] [clang] Warn on umask() argument bits outside 0777 (PR #198130)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Builtins.td:3868:7: error: Unknown Type: mode_t
> ```let Prototype = "mode_t(mode_t)";```

Today I learned that even if the signature is ignored, it still needs to be valid. Let's use comments for the prototype instead:
```
let Prototype = ""; // mode_t(mode_t); mode_t is target-specific
```

https://github.com/llvm/llvm-project/pull/198130