gdb/python: add gdb.Frame.__repr__() method
Add a gdb.Frame.__repr__() method. Before this patch we would see
output like this:
(gdb) pi
>>> gdb.selected_frame()
<gdb.Frame object at 0x7fa8cc2df270>
After this patch, we now see:
(gdb) pi
>>> gdb.selected_frame()
<gdb.Frame level=0 frame-id={stack=0x7ffff7da0ed0,code=0x000000000040115d,!special}>
More verbose, but, I hope, more useful.
If the gdb.Frame becomes invalid, then we will see:
(gdb) pi
>>> invalid_frame_variable
<gdb.Frame (invalid)>
which is inline with how other invalid objects are displayed.
Approved-By: Tom Tromey <tom@tromey.com>