[clang] [clang][SSAF] Optionally skip system-header contributors (PR #205446)

[Jan Korous via cfe-commits]

jkorous-apple wrote:

Pending more github infra issues the checks should start automatically.
But I currently see this message:
```
Checks awaiting conflict resolution
...
This branch has conflicts that must be resolved
```

@t-rasmud can you rebase the branch on `llvm/main`?

https://github.com/llvm/llvm-project/pull/205446