diagnostic improvement: missing-typename diagnostic

[hokein]

Given the following code:

template<typename T1>
struct A1 {
  template<typename T2>
  struct A2 { T2 t; };
};

template <typename K>
void s(K k) {
   A1<K>::A2 a(2); // missing a `typename` at head!
}

void s() {
  s<double>(1.0);
}

Diagnostics emitted by clang are not quite useful, it would be nice to emit a "missing typename" diagnostic (both gcc and msvc do it for this case).

<source>:15:13: error: expected ';' after expression
   15 |    A1<K>::A2 a(2); // missing a typenamea at head!
      |             ^
      |             ;
<source>:15:14: error: use of undeclared identifier 'a'
   15 |    A1<K>::A2 a(2); // missing a typename at head!
      |              ^
<source>:15:11: error: use of class template 'A2' requires template arguments
   15 |    A1<K>::A2 a(2); // missing a typename at head!
      |           ^
<source>:19:3: note: in instantiation of function template specialization 's<double>' requested here
   19 |   s<double>(1.0);
      |   ^
<source>:10:10: note: template is declared here
    9 |   template<typename T2>
      |   ~~~~~~~~~~~~~~~~~~~~~
   10 |   struct A2 { T2 t; };
      |          ^