Tweak the way that is_a is implemented

At the moment, class hierarchies that use is_a are expected
to define specialisations like:

  template <>
  template <>
  inline bool
  is_a_helper <cgraph_node *>::test (symtab_node *p)
  {
    return p->type == SYMTAB_FUNCTION;
  }

But this doesn't scale well to larger hierarchies, because it only
defines ::test for an argument that is exactly “symtab_node *”
(and not for example “const symtab_node *” or something that
comes between cgraph_node and symtab_node in the hierarchy).

For example:

  struct A { int x; };
  struct B : A {};
  struct C : B {};

  template <>
  template <>
  inline bool
  is_a_helper <C *>::test (A *a)
  {
    return a->x == 1;
  }

  bool f(B *b) { return is_a<C *> (b); }

gives:

  warning: inline function ‘static bool is_a_helper<T>::test(U*) [with U = B; T = C*]’ used but never defined

and:

  bool f(const A *a) { return is_a<const C *> (a); }

gives:

  warning: inline function ‘static bool is_a_helper<T>::test(U*) [with U = const A; T = const C*]’ used but never defined

This patch instead allows is_a to be implemented by specialising
is_a_helper as a whole, for example:

  template<>
  struct is_a_helper<C *> : static_is_a_helper<C *>
  {
    static inline bool test (const A *a) { return a->x == 1; }
  };

It also adds a general specialisation of is_a_helper for const
pointers.  Together, this makes both of the above examples work.

gcc/
	* is-a.h (reinterpret_is_a_helper): New class.
	(static_is_a_helper): Likewise.
	(is_a_helper): Inherit from reinterpret_is_a_helper.
	(is_a_helper<const T *>): New specialization.

gcc/is-a.h

@@ -116,9 +116,30 @@ the connection between the types has not been made.  See below.
 
 EXTENDING THE GENERIC TYPE FACILITY
 
-Each connection between types must be made by defining a specialization of the
+Method 1
-template member function 'test' of the template class 'is_a_helper'.  For
+--------
-example,
+
+If DERIVED is derived from BASE, and if BASE contains enough information
+to determine whether an object is actually an instance of DERIVED,
+then you can make the above routines work for DERIVED by defining
+a specialization of is_a_helper such as:
+
+  template<>
+  struct is_a_helper<DERIVED *> : static_is_a_helper<DERIVED *>
+  {
+    static inline bool test (const BASE *p) { return ...; }
+  };
+
+This test function should return true if P is an instanced of DERIVED.
+This on its own is enough; the comments below for method 2 do not apply.
+
+Method 2
+--------
+
+Alternatively, if two types are connected in ways other than C++
+inheritance, each connection between them must be made by defining a
+specialization of the template member function 'test' of the template
+class 'is_a_helper'.  For example,
 
   template <>
   template <>
@@ -145,15 +166,52 @@ when needed may result in a crash.  For example,
 #ifndef GCC_IS_A_H
 #define GCC_IS_A_H
 
+/* A base class that specializations of is_a_helper can use if casting
+   U * to T is simply a reinterpret_cast.  */
+
+template <typename T>
+struct reinterpret_is_a_helper
+{
+  template <typename U>
+  static inline T cast (U *p) { return reinterpret_cast <T> (p); }
+};
+
+/* A base class that specializations of is_a_helper can use if casting
+   U * to T is simply a static_cast.  This is more type-safe than
+   reinterpret_is_a_helper.  */
+
+template <typename T>
+struct static_is_a_helper
+{
+  template <typename U>
+  static inline T cast (U *p) { return static_cast <T> (p); }
+};
+
 /* A generic type conversion internal helper class.  */
 
 template <typename T>
-struct is_a_helper
+struct is_a_helper : reinterpret_is_a_helper<T>
 {
   template <typename U>
   static inline bool test (U *p);
+};
+
+/* Reuse the definition of is_a_helper<T *> to implement
+   is_a_helper<const T *>.  */
+
+template <typename T>
+struct is_a_helper<const T *>
+{
   template <typename U>
-  static inline T cast (U *p);
+  static inline const T *cast (const U *p)
+  {
+    return is_a_helper<T *>::cast (const_cast <U *> (p));
+  }
+  template <typename U>
+  static inline bool test (const U *p)
+  {
+    return is_a_helper<T *>::test (p);
+  }
 };
 
 /* Note that we deliberately do not define the 'test' member template.  Not
@@ -161,19 +219,6 @@ struct is_a_helper
    not been defined, rather than a run-time error.  See the discussion above
    for when to define this member.  */
 
-/* This is the generic implementation for casting from one type to another.
-   Do not use this routine directly; it is an internal function.  See the
-   discussion above for when to define this member.  */
-
-template <typename T>
-template <typename U>
-inline T
-is_a_helper <T>::cast (U *p)
-{
-  return reinterpret_cast <T> (p);
-}
-
-
 /* The public interface.  */
 
 /* A generic test for a type relationship.  See the discussion above for when