Rename base_breakpoint -> code_breakpoint

Rename base_breakpoint -> code_breakpoint

Even after the previous patches reworking the inheritance of several
breakpoint types, the present breakpoint hierarchy looks a bit
surprising, as we have "breakpoint" as the superclass, and then
"base_breakpoint" inherits from "breakpoint".  Like so, simplified:

   breakpoint
       base_breakpoint
          ordinary_breakpoint
	  internal_breakpoint
	  momentary_breakpoint
	  ada_catchpoint
	  exception_catchpoint
       tracepoint
       watchpoint
       catchpoint
	  exec_catchpoint
	  ...

The surprising part to me is having "base_breakpoint" being a subclass
of "breakpoint".  I'm just refering to naming here -- I mean, you'd
expect that it would be the top level baseclass that would be called
"base".

Just flipping the names of breakpoint and base_breakpoint around
wouldn't be super great for us, IMO, given we think of every type of
*point as a breakpoint at the user visible level.  E.g., "info
breakpoints" shows watchpoints, tracepoints, etc.  So it makes to call
the top level class breakpoint.

Instead, I propose renaming base_breakpoint to code_breakpoint.  The
previous patches made sure that all code breakpoints inherit from
base_breakpoint, so it's fitting.  Also, "code breakpoint" contrasts
nicely with a watchpoint also being typically known as a "data
breakpoint".

After this commit, the resulting hierarchy looks like:

   breakpoint
       code_breakpoint
          ordinary_breakpoint
	  internal_breakpoint
	  momentary_breakpoint
	  ada_catchpoint
	  exception_catchpoint
       tracepoint
       watchpoint
       catchpoint
	  exec_catchpoint
	  ...

... which makes a lot more sense to me.

I've left this patch as last in the series in case people want to
bikeshed on the naming.

"code" has a nice property that it's exactly as many letters as
"base", so this patch didn't require any reindentation.  :-)

Change-Id: Id8dc06683a69fad80d88e674f65e826d6a4e3f66