[clang] [Clang][C2y] Add support for if declarations (N3356 paper) (PR #198244)

[Aaron Ballman via cfe-commits]

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@@ -1970,6 +2024,13 @@ Sema::ConditionResult Parser::ParseCXXCondition(StmtResult *InitStmt,
   DeclSpec DS(AttrFactory);
   ParseSpecifierQualifierList(DS, AS_none, DeclSpecContext::DSC_condition);
 
+  // The Declarator's DeclarationAttrs only accepts [[]] and keyword attributes;
+  // move any GNU attributes onto the DeclSpec instead.
+  if (!getLangOpts().CPlusPlus)
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AaronBallman wrote:

This seems pretty suspicious to me, can you explain why it's needed a bit more? CC @erichkeane 

https://github.com/llvm/llvm-project/pull/198244