[clang] [Clang] disallow use of attributes before extern template declarations (PR #136328)

[Erich Keane via cfe-commits]

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@@ -1049,6 +1049,7 @@ Parser::ParseExternalDeclaration(ParsedAttributes &Attrs,
 
   case tok::kw_extern:
     if (getLangOpts().CPlusPlus && NextToken().is(tok::kw_template)) {
+      ProhibitAttributes(Attrs);
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erichkeane wrote:

It is prohibited by the grammar for standard attributes, but it seems like it could be sensible to allow non-standard attributes?  WDYT?

https://github.com/llvm/llvm-project/pull/136328