[clang] [Clang] add fix-it hints for unknown attributes (PR #141305)

[Oleksandr T. via cfe-commits]

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@@ -1842,8 +1842,7 @@ void Parser::ProhibitCXX11Attributes(ParsedAttributes &Attrs,
       continue;
     if (AL.getKind() == ParsedAttr::UnknownAttribute) {
       if (WarnOnUnknownAttrs)
-        Diag(AL.getLoc(), diag::warn_unknown_attribute_ignored)
-            << AL << AL.getRange();
+        Actions.DiagnoseUnknownAttribute(AL);
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a-tarasyuk wrote:

I'll check, though it seems that unknown attributes should also be marked as invalid

https://github.com/llvm/llvm-project/pull/141305