[clang] [Attributes] Support Attributes being declared as supporting an experimental late parsing mode "extension" (PR #88596)

[Aaron Ballman via cfe-commits]

================
@@ -592,6 +592,48 @@ class AttrSubjectMatcherAggregateRule<AttrSubject subject> {
 
 def SubjectMatcherForNamed : AttrSubjectMatcherAggregateRule<Named>;
 
+// Late Attribute parsing mode enum
+class LateAttrParseKind <int val> {
+  int Kind = val;
+}
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AaronBallman wrote:

```suggestion
// Enumeration specifying what kind of behavior should be used for late
// parsing of attributes.
class LateAttrParseKind<int Val> {
  int Kind = Val;
}
```


https://github.com/llvm/llvm-project/pull/88596