[clang] [clang] Add support for [[clang::diagnose_if(...)]] and [[clang::enable_if(...)]] syntax (PR #225900)

[via cfe-commits]

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@@ -1938,7 +1902,7 @@ def EnableIf : InheritableAttr {
   // Does not have a [[]] spelling because this attribute requires the ability
   // to parse function arguments but the attribute is not written in the type
   // position.
-  let Spellings = [GNU<"enable_if">];
+  let Spellings = [Clang<"enable_if">];
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higher-performance wrote:

(I do get that it's ugly to conflate things from a design standpoint -- I understand why you feel strongly about that and I don't feel strongly enough about this to suggest it's worth a wider discussion, so I'm fine leaving this as is. At this point I'm just trying to figure out if it could theoretically cause an actual problem for developers, or if it's solely a language design ugliness.)

https://github.com/llvm/llvm-project/pull/225900