c++: simplify member template substitution

c++: simplify member template substitution

I noticed that for member class templates of a class template we were
unnecessarily substituting both the template and its type.  Avoiding that
duplication speeds compilation of this silly testcase from ~12s to ~9s on my
laptop.  It's unlikely to make a difference on any real code, but the
simplification is also nice.

We still need to clear CLASSTYPE_USE_TEMPLATE on the partial instantiation
of the template class, but it makes more sense to do that in
tsubst_template_decl anyway.

  #define NC(X)					\
    template <class U> struct X##1;		\
    template <class U> struct X##2;		\
    template <class U> struct X##3;		\
    template <class U> struct X##4;		\
    template <class U> struct X##5;		\
    template <class U> struct X##6;
  #define NC2(X) NC(X##a) NC(X##b) NC(X##c) NC(X##d) NC(X##e) NC(X##f)
  #define NC3(X) NC2(X##A) NC2(X##B) NC2(X##C) NC2(X##D) NC2(X##E)
  template <int I> struct A
  {
    NC3(am)
  };
  template <class...Ts> void sink(Ts...);
  template <int...Is> void g()
  {
    sink(A<Is>()...);
  }
  template <int I> void f()
  {
    g<__integer_pack(I)...>();
  }
  int main()
  {
    f<1000>();
  }

gcc/cp/ChangeLog:

	* pt.cc (instantiate_class_template): Skip the RECORD_TYPE
	of a class template.
	(tsubst_template_decl): Clear CLASSTYPE_USE_TEMPLATE.