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

[Aaron Ballman via cfe-commits]

================
@@ -1822,28 +1822,100 @@ void WriteSemanticSpellingSwitch(const std::string &VarName,
   OS << "  }\n";
 }
 
+enum class LateAttrParseKind { Never = 0, Standard = 1, ExperimentalExt = 2 };
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AaronBallman wrote:

```suggestion
// Note: these values need to match the values used by LateAttrParseKind in Attr.td.
enum class LateAttrParseKind { Never = 0, Standard = 1, ExperimentalExt = 2 };
```

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