Add missing declarations of explicit template instantiations. (#86964)

Found with -Wundefined-func-template.

Author: tkoeppe

clang/lib/AST/Interp/ByteCodeStmtGen.h

@@ -82,6 +82,7 @@ class ByteCodeStmtGen final : public ByteCodeExprGen<Emitter> {
   OptLabelTy DefaultLabel;
 };
 
+extern template class ByteCodeStmtGen<ByteCodeEmitter>;
 extern template class ByteCodeExprGen<EvalEmitter>;
 
 } // namespace interp