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

[Balázs Benics via cfe-commits]

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@@ -41,9 +41,18 @@ class SSAFOptions {
   LLVM_PREFERRED_TYPE(bool)
   unsigned ShowFormats : 1;
 
+  /// Extract from system-header declarations during SSAF contributor
+  /// enumeration. Defaults to true to preserve the original behavior.
+  /// Cleared by `--ssaf-no-extract-from-system-headers` (negative-marshalled
+  /// flag) when the caller wants to scope contributor enumeration to
+  /// user-source decls.
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steakhal wrote:

Personally, this seemed redundant. And I think we should follow the previously set practice/format for spelling the flag.
```suggestion
  /// Controlled by: --ssaf-no-extract-from-system-headers
```

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