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

[Erich Keane via cfe-commits]

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

ALSO: I checked and this should only prevent `regular` keywords-as-attributes, which seem to only be ARM based ones, so this won't affect things like __alignas  or calling conventions/etc.

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