gdb/python: add gdb.InferiorThread.__repr__() method

gdb/python: add gdb.InferiorThread.__repr__() method

Add a gdb.InferiorThread.__repr__() method.  Before this patch we
would see output like this:

  (gdb) pi
  >>> gdb.selected_thread()
  <gdb.InferiorThread object at 0x7f4dcc49b970>

After this patch, we now see:

  (gdb) pi
  >>> gdb.selected_thread()
  <gdb.InferiorThread id=1.2 target-id="Thread 0x7ffff7da1700 (LWP 458134)">

More verbose, but, I hope, more useful.

If the gdb.InferiorThread becomes invalid, then we will see:

  (gdb) pi
  >>> invalid_thread_variable
  <gdb.InferiorThread (invalid)>

Which is inline with how other invalid objects are displayed.

Approved-By: Tom Tromey <tom@tromey.com>