gdb: move frame_info_ptr method implementations to frame-info.c

gdb: move frame_info_ptr method implementations to frame-info.c

I don't see any particular reason why the implementations of the
frame_info_ptr object are in the header file.  It only seems to add some
complexity.  Since we can't include frame.h in frame-info.h, we have to
add declarations of functions defined in frame.c, in frame-info.h.  By
moving the implementations to a new frame-info.c, we can avoid that.

Change-Id: I435c828f81b8a3392c43ef018af31effddf6be9c
Reviewed-By: Bruno Larsen <blarsen@redhat.com>
Reviewed-By: Tom Tromey <tom@tromey.com>