[clang] [clang] Implement `__is_layout_compatible` (PR #81506)

[via cfe-commits]

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@@ -1717,6 +1717,7 @@ void Parser::ParseClassSpecifier(tok::TokenKind TagTokKind,
           tok::kw___is_fundamental,
           tok::kw___is_integral,
           tok::kw___is_interface_class,
+          tok::kw___is_layout_compatible,
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cor3ntin wrote:

I don't think this is necessary. It exists for compatibility with old GCCs that would not have had the notion of layout compatibility 

(and I wonder if we could get rid of the whole switch ultimately @AaronBallman)

https://github.com/llvm/llvm-project/pull/81506