[clang] [clang] Improve `_Alignas` on a `struct` declaration diagnostic (PR #65638)

[Jerin Philip via cfe-commits]

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@@ -192,6 +192,13 @@ class AttributeCommonInfo {
 
   bool isC23Attribute() const { return SyntaxUsed == AS_C23; }
 
+  bool isAlignas() const {
+    // In the current state of code, IsAlignas is only configured to return
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jerinphilip wrote:

This refers to the variable `IsAlignAs` (not the parent function `isAlignas`), and the sentence holds for the current state of code. I believe I'm trying to explain why I cannot use `IsAlignas`'s current behaviour here (as it is more invasive). Normally I'd expect this function to be a `const` accessor to a private member per programming conventions, which is not the case here, hence a comment.

I'm sensing the review comment confusion indicates I need to communicate this better in source comments. Suggestions welcome. 

https://github.com/llvm/llvm-project/pull/65638