gdb: make bp_locations an std::vector

gdb: make bp_locations an std::vector

Change the type of the global location list, bp_locations, to be an
std::vector.

Adjust the users to deal with that, mostly in an obvious way by using
.data() and .size().  The user where it's slightly less obvious is
update_global_location_list.  There, we std::move the old location list
out of the global vector into a local variable.  The code to fill the
new location list gets simpler, as it's now simply using .push_back(),
no need to count the locations beforehand.

In the rest of update_global_location_list, the code is adjusted to work
with indices instead of `bp_location **`, to iterate on the location
list.  I believe it's a bit easier to understand this way.  But more
importantly, when we build with _GLIBCXX_DEBUG, the operator[] of the
vector does bound checking, so we will know if we ever access past a
vector size (which we won't if we access by raw pointer).  I think that
work can further be done to make that function easier to understand,
notably find better names than "loc" and "loc2" for variables, but
that's work for later.

gdb/ChangeLog:

	* breakpoint.c (bp_locations): Change to std::vector, update all
	users.
	(bp_locations_count): Remove.
	(update_global_location_list): Change to work with indices
	rather than bp_location**.

Change-Id: I193ce40f84d5dc930fbab8867cf946e78ff0df0b