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

[via cfe-commits]

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@@ -169,6 +169,15 @@ def ext_c2y_generic_with_type_arg : Extension<
 def warn_c2y_compat_generic_with_type_arg : Warning<
   "passing a type argument as the first operand to '_Generic' is incompatible "
   "with C standards before C2y">, InGroup<CPre2yCompat>, DefaultIgnore;
+def warn_c2y_empty_declaration_statement : Warning<
+  "empty declaration statement of '%select{if|switch}0' has no effect">,
+  InGroup<C2y>;
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Sirraide wrote:

The `C2y` group is specifically for compatibility diagnostics; general diagnostics that just happen to be for a C2y feature should not be in this group.

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