gdb: add breakpoint::locations method

gdb: add breakpoint::locations method

Add the breakpoint::locations method, which returns a range that can be
used to iterate over a breakpoint's locations.  This shortens

  for (bp_location *loc = b->loc; loc != nullptr; loc = loc->next)

into

  for (bp_location *loc : b->locations ())

Change all the places that I found that could use it.

gdb/ChangeLog:

	* breakpoint.h (bp_locations_range): New.
	(struct breakpoint) <locations>: New.  Use where possible.

Change-Id: I1ba2f7d93d57e544e1f8609124587dcf2e1da037