gdb: Use a typedef's scoped type name to identify local typedefs

GDB prints the wrong type for typedefs in case there is another typedef
available for the same raw type (gdb/16040).  The reason is that the
current hashmap based substitution mechanism always compares the target
type of a typedef and not its scoped name.

The original output of GDB for a program like

~~~~
namespace ns
{
  typedef double scoped_double;
}

typedef double global_double;

class TypedefHolder
{
public:
  double a;
  ns::scoped_double b;
  global_double c;

private:
  typedef double class_double;
  class_double d;

  double method1(ns::scoped_double) { return 24.0; }
  double method2(global_double) { return 24.0; }
};

int main()
{
  TypedefHolder th;
  return 0;
}
~~~~

is
~~~~

(gdb) b 27
Breakpoint 1 at 0x1131: file TypedefHolder.cc, line 27.
(gdb) r
Starting program: /tmp/typedefholder

Breakpoint 1, main () at TypedefHolder.cc:27
27	  return 0;
(gdb) ptype th
type = class TypedefHolder {
  public:
    class_double a;
    class_double b;
    class_double c;
  private:
    class_double d;

    class_double method1(class_double);
    class_double method2(class_double);

    typedef double class_double;
}
~~~~

Basically all attributes of a class which have the raw type "double" are
substituted by "class_double".

With the patch the output is the following

~~~~
type = class TypedefHolder {
  public:
    double a;
    ns::scoped_double b;
    global_double c;
  private:
    class_double d;

    double method1(ns::scoped_double);
    double method2(global_double);

    typedef double class_double;
}
~~~~

gdb/typeprint.c

@@ -201,9 +201,8 @@ static hashval_t
 hash_typedef_field (const void *p)
 {
   const struct decl_field *tf = (const struct decl_field *) p;
-  struct type *t = check_typedef (tf->type);
 
-  return htab_hash_string (TYPE_SAFE_NAME (t));
+  return htab_hash_string (TYPE_SAFE_NAME (tf->type));
 }
 
 /* An equality function for a typedef field.  */