[clang] [PAC][clang] Define `PointerAuthQualifier` and `PointerAuthenticationMode` (PR #84384)

[Aaron Ballman via cfe-commits]

================
@@ -194,19 +358,27 @@ class Qualifiers {
     FastMask = (1 << FastWidth) - 1
   };
 
+  Qualifiers() : Mask(0), PtrAuth() {}
+
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AaronBallman wrote:

```suggestion
```
Not needed; the default ctor will be automatically called and `Mask` has an inline initializer.

https://github.com/llvm/llvm-project/pull/84384