gdb: add frame_id::user_created_p

gdb: add frame_id::user_created_p

Later in this series, we'll need to differentiate frame ids for regular
frames (obtained from the target state and unwinding from it) vs frame
ids for user-created frames (created with create_new_frame).  Add the
frame_id::user_created_p field to indicate a frame is user-created, and
set it in create_new_frame.

The field is otherwise not used yet, so not changes in behavior are
expected.

Change-Id: I60de3ce581ed01bf0fddb30dff9bd932840120c3
Reviewed-By: Bruno Larsen <blarsen@redhat.com>