[clang] [Clang] Improve EmitClangAttrSpellingListIndex (PR #114899)

[Erich Keane via cfe-commits]

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@@ -67,6 +67,16 @@ class AttributeCommonInfo {
     IgnoredAttribute,
     UnknownAttribute,
   };
+  enum Scope {
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erichkeane wrote:

One concern of this is that we should make sure to diagnose if we don't find the value.  Hoping to find that below :) 

https://github.com/llvm/llvm-project/pull/114899