Notify Python when breakpoint symbol changes

A DAP user noticed that breakpoints set by address were never updated
to show their location after the DAP launch request.  It turns out
that gdb does not emit the breakpoint-modified event when this sort of
breakpoint is updated.

This patch changes gdb to notify the breakpoint-modified observer when
a breakpoint location's symbol changes.  This in turn causes the DAP
event to be emitted.

Reviewed-by: Keith Seitz <keiths@redhat.com>