Create private_thread_info hierarchy

There are multiple definitions of the private_thread_info structure
compiled in the same GDB build.  Because of the one definition rule, we
need to change this if we want to be able to make them non-POD (e.g. use
std::vector fields).  This patch creates a class hierarchy, with
private_thread_info being an abstract base class, and all the specific
implementations inheriting from it.

In order to poison XNEW/xfree for non-POD types, it is also needed to
get rid of the xfree in thread_info::~thread_info, which operates on an
opaque type.  This is replaced by thread_info::priv now being a
unique_ptr, which calls the destructor of the private_thread_info
subclass when the thread is being destroyed.

Including gdbthread.h from darwin-nat.h gave these errors:

/Users/simark/src/binutils-gdb/gdb/gdbthread.h:609:3: error: must use 'class' tag to refer to type 'thread_info' in this scope
  thread_info *m_thread;
  ^
  class
/usr/include/mach/thread_act.h:240:15: note: class 'thread_info' is hidden by a non-type declaration of 'thread_info' here
kern_return_t thread_info
              ^

It turns out that there is a thread_info function in the Darwin/XNU/mach API:

  http://web.mit.edu/darwin/src/modules/xnu/osfmk/man/thread_info.html

Therefore, I had to add the class keyword at a couple of places in gdbthread.h,
I don't really see a way around it.

gdb/ChangeLog:

	* gdbthread.h (private_thread_info): Define structure type, add
	virtual pure destructor.
	(thread_info) <priv>: Change type to unique_ptr.
	<private_dtor>: Remove.
	* thread.c (add_thread_with_info): Adjust to use of unique_ptr.
	(private_thread_info::~private_thread_info): Provide default
	implementation.
	(thread_info::~thread_info): Don't call private_dtor nor
	manually free priv.
	* aix-thread.c (private_thread_info): Rename to ...
	(aix_thread_info): ... this.
	(get_aix_thread_info): New.
	(sync_threadlists): Adjust.
	(iter_tid): Adjust.
	(aix_thread_resume): Adjust.
	(aix_thread_fetch_registers): Adjust.
	(aix_thread_store_registers): Adjust.
	(aix_thread_extra_thread_info): Adjust.
	* darwin-nat.h (private_thread_info): Rename to ...
	(darwin_thread_info): ... this.
	(get_darwin_thread_info): New.
	* darwin-nat.c (darwin_init_thread_list): Adjust.
	(darwin_check_new_threads): Adjust.
	(thread_info_from_private_thread_info): Adjust.
	* linux-thread-db.c (private_thread_info): Rename to ...
	(thread_db_thread_info): ... this, initialize fields.
	(get_thread_db_thread_info): New.
	<dying>: Change type to bool.
	(update_thread_state): Adjust to type rename.
	(record_thread): Adjust to type rename an use of unique_ptr.
	(thread_db_pid_to_str): Likewise.
	(thread_db_extra_thread_info): Likewise.
	(thread_db_thread_handle_to_thread_info): Likewise.
	(thread_db_get_thread_local_address): Likewise.
	* nto-tdep.h (private_thread_info): Rename to ...
	(nto_thread_info): ... this, initialize fields.
	(get_nto_thread_info): New.
	<name>: Change type to std::string.
	* nto-tdep.c (nto_extra_thread_info): Adjust to type rename and
	use of unique_ptr.
	* nto-procfs.c (update_thread_private_data_name): Adjust to
	std::string change, allocate nto_private_thread_info with new.
	(update_thread_private_data): Adjust to unique_ptr.
	* remote.c (private_thread_info): Rename to ...
	(remote_thread_info): ... this, initialize data members with
	default values.
	<extra, name>: Change type to std::string.
	<thread_handle>: Change type to non-pointer.
	(free_private_thread_info): Remove.
	(get_private_info_thread): Rename to...
	(get_remote_thread_info): ... this, change return type, adjust to
	use of unique_ptr, use remote_thread_info constructor.
	(remote_add_thread): Adjust.
	(get_private_info_ptid): Rename to...
	(get_remote_thread_info): ...this, change return type.
	(remote_thread_name): Use get_remote_thread_info, adjust to
	change to std::string.
	(struct thread_item) <~thread_item>: Remove.
	<thread_handle>: Make non pointer.
	(start_thread): Adjust to thread_item::thread_handle type
	change.
	(remote_update_thread_list): Adjust to type name change, move
	strings from temporary to long-lived object instead of
	duplicating.
	(remote_threads_extra_info): Use get_remote_thread_info.
	(process_initial_stop_replies): Likewise.
	(resume_clear_thread_private_info): Likewise.
	(remote_resume): Adjust to type name change.
	(remote_commit_resume): Use get_remote_thread_info.
	(process_stop_reply): Adjust to type name change.
	(remote_stopped_by_sw_breakpoint): Use get_remote_thread_info.
	(remote_stopped_by_hw_breakpoint): Likewise.
	(remote_stopped_by_watchpoint): Likewise.
	(remote_stopped_data_address): Likewise.
	(remote_core_of_thread): Likewise.
	(remote_thread_handle_to_thread_info): Use
	get_private_info_thread, adjust to thread_handle field type
	change.

gdb/gdbthread.h

@@ -179,6 +179,12 @@ typedef struct value *value_ptr;
 DEF_VEC_P (value_ptr);
 typedef VEC (value_ptr) value_vec;
 
+/* Base class for target-specific thread data.  */
+struct private_thread_info
+{
+  virtual ~private_thread_info () = 0;
+};
+
 /* Threads are intrusively refcounted objects.  Being the
    user-selected thread is normally considered an implicit strong
    reference and is thus not accounted in the refcount, unlike
@@ -345,11 +351,7 @@ public:
   struct frame_id initiating_frame = null_frame_id;
 
   /* Private data used by the target vector implementation.  */
-  struct private_thread_info *priv = NULL;
-
-  /* Function that is called to free PRIVATE.  If this is NULL, then
-     xfree will be called on PRIVATE.  */
-  void (*private_dtor) (struct private_thread_info *) = NULL;
+  std::unique_ptr<private_thread_info> priv;
 
   /* Branch trace information for this thread.  */
   struct btrace_thread_info btrace {};
@@ -604,7 +606,9 @@ public:
   DISABLE_COPY_AND_ASSIGN (scoped_restore_current_thread);
 
 private:
-  thread_info *m_thread;
+  /* Use the "class" keyword here, because of a clash with a "thread_info"
+     function in the Darwin API.  */
+  class thread_info *m_thread;
   inferior *m_inf;
   frame_id m_selected_frame_id;
   int m_selected_frame_level;
@@ -683,7 +687,7 @@ extern void print_selected_thread_frame (struct ui_out *uiout,
    Selects thread THR.  TIDSTR is the original string the thread ID
    was parsed from.  This is used in the error message if THR is not
    alive anymore.  */
-extern void thread_select (const char *tidstr, thread_info *thr);
+extern void thread_select (const char *tidstr, class thread_info *thr);
 
 extern struct thread_info *thread_list;