Move add_location(sal) to base_breakpoint

Move add_location(sal) to base_breakpoint

After the previous patches, only base_breakpoint subclasses use
add_location(sal), so we can move it to base_breakpoint (a.k.a. base
class for code breakpoints).

This requires a few casts here and there, but always at spots where
you can see from context what the breakpoint's type actually is.

I inlined new_single_step_breakpoint into its only caller exactly for
this reason.

I did try to propagate more use of base_breakpoint to avoid casts, but
that turned out unwieldy for this patch.

Change-Id: I49d959322b0fdce5a88a216bb44730fc5dd7c6f8