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

[Aaron Ballman via cfe-commits]

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@@ -220,8 +230,28 @@ void Parser::ParseGNUAttributes(ParsedAttributes &Attrs,
         continue;
       }
 
+      bool LateParse = false;
+      if (!LateAttrs)
+        LateParse = false;
+      else {
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AaronBallman wrote:

Can remove a level of indentation here by making this an `else if` followed by an `else`.

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