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#gcc-mirror#1;		\
    template <class U> struct X#gcc-mirror#2;		\
    template <class U> struct X#gcc-mirror#3;		\
    template <class U> struct X#gcc-mirror#4;		\
    template <class U> struct X#gcc-mirror#5;		\
    template <class U> struct X#gcc-mirror#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.

gcc/cp/pt.cc

@@ -12285,41 +12285,20 @@ instantiate_class_template (tree type)
 		   Ignore it; it will be regenerated when needed.  */
 		continue;
 
-	      bool class_template_p = (TREE_CODE (t) != ENUMERAL_TYPE
-				       && TYPE_LANG_SPECIFIC (t)
-				       && CLASSTYPE_IS_TEMPLATE (t));
-
-	      /* If the member is a class template, then -- even after
-		 substitution -- there may be dependent types in the
-		 template argument list for the class.  We increment
-		 PROCESSING_TEMPLATE_DECL so that dependent_type_p, as
-		 that function will assume that no types are dependent
-		 when outside of a template.  */
-	      if (class_template_p)
-		++processing_template_decl;
+	      /* If the member is a class template, we've
+		 already substituted its type.  */
+	      if (CLASS_TYPE_P (t)
+		  && CLASSTYPE_IS_TEMPLATE (t))
+		continue;
+
 	      tree newtag = tsubst (t, args, tf_error, NULL_TREE);
-	      if (class_template_p)
-		--processing_template_decl;
 	      if (newtag == error_mark_node)
 		continue;
 
 	      if (TREE_CODE (newtag) != ENUMERAL_TYPE)
 		{
 		  tree name = TYPE_IDENTIFIER (t);
 
-		  if (class_template_p)
-		    /* Unfortunately, lookup_template_class sets
-		       CLASSTYPE_IMPLICIT_INSTANTIATION for a partial
-		       instantiation (i.e., for the type of a member
-		       template class nested within a template class.)
-		       This behavior is required for
-		       maybe_process_partial_specialization to work
-		       correctly, but is not accurate in this case;
-		       the TAG is not an instantiation of anything.
-		       (The corresponding TEMPLATE_DECL is an
-		       instantiation, but the TYPE is not.) */
-		    CLASSTYPE_USE_TEMPLATE (newtag) = 0;
-
 		  /* Now, install the tag.  We don't use pushtag
 		     because that does too much work -- creating an
 		     implicit typedef, which we've already done.  */
@@ -14750,7 +14729,10 @@ tsubst_template_decl (tree t, tree args, tsubst_flags_t complain,
       /* For a partial specialization, we need to keep pointing to
 	 the primary template.  */
       if (!DECL_TEMPLATE_SPECIALIZATION (t))
-	CLASSTYPE_TI_TEMPLATE (inner) = r;
+	{
+	  CLASSTYPE_TI_TEMPLATE (inner) = r;
+	  CLASSTYPE_USE_TEMPLATE (inner) = 0;
+	}
 
       DECL_TI_ARGS (r) = CLASSTYPE_TI_ARGS (inner);
       inner = TYPE_MAIN_DECL (inner);