[clang] [Attributes] Support Attributes being declared as only supporting late parsing when passing an experimental feature flag (PR #88596)

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@@ -1822,28 +1822,106 @@ void WriteSemanticSpellingSwitch(const std::string &VarName,
   OS << "  }\n";
 }
 
+enum class LateAttrParseKind {
+  LateAttrParsingNever = 0,
+  LateAttrParsingAlways = 1,
+  LateAttrParsingExperimentalOnly = 2
+};
+
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Sirraide wrote:

If it’s just for the macro here, then one option would also be to use token pasting to concatenate `LateAttrParsing` and the name of the enumerator.

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