[istl][multitype] fix argument deduction for clang

we have to preceed the variadic parameter by T1, in order to fix the deduction. The problem is
that (for what ever reason) otherwise the type pack can not be detected properly.

dune/istl/matrixutils.hh

@@ -119,11 +119,11 @@ namespace Dune
   template<typename FirstRow, typename... Args>
   class MultiTypeBlockMatrix;
 
-  template<typename... Args, std::size_t blocklevel, std::size_t l>
-  struct CheckIfDiagonalPresent<MultiTypeBlockMatrix<Args...>,
+  template<std::size_t blocklevel, std::size_t l, typename T1, typename... Args>
+  struct CheckIfDiagonalPresent<MultiTypeBlockMatrix<T1,Args...>,
       blocklevel,l>
   {
-    typedef MultiTypeBlockMatrix<Args...> Matrix;
+    typedef MultiTypeBlockMatrix<T1,Args...> Matrix;
 
     /**
      * @brief Check whether the a matrix has diagonal values