Fixed several problems with watchpoint expressions.

- First, the watchpoint size was being cast to the
  wrong type.  This is primarily cosmetic, but
  annoying.

- Second, the options for the watchpoint command
  were not being initialized correctly, which led
  to the watchpoint size sometimes having absurdly
  large values.  This caused watchpoints to fail to
  be set in some cases.

<rdar://problem/12658775>