Add the ability to set breakpoints with conditions, commands, etc,
in the "dummy-target".  The dummy target breakpoints prime all future
targets.  Breakpoints set before any target is created (e.g. breakpoints
in ~/.lldbinit) automatically get set in the dummy target.  You can also
list, add & delete breakpoints from the dummy target using the "-D" flag,
which is supported by most of the breakpoint commands.

This removes a long-standing wart in lldb...

<rdar://problem/10881487>
SelectionDAG switch lowering: Replace unreachable default with most popular case.

This can significantly reduce the size of the switch, allowing for more
efficient lowering.

I also worked with the idea of exploiting unreachable defaults by
omitting the range check for jump tables, but always ended up with a
non-neglible binary size increase. It might be worth looking into some more.

SimplifyCFG currently does this transformation, but I'm working towards changing
that so we can optimize harder based on unreachable defaults.

Differential Revision: http://reviews.llvm.org/D6510