Update CrashCatcher.h

* I moved the magic STACK_SENTINEL value into the public CrashCatcher
  header file so that CrashDebug wouldn't need to use the private
  header to check the last word in the dump to see if a stack overflow
  was detected.
* Added a CrashCatcherMemoryRegionInfo structure which is similar to
  CrashCatcherMemoryRegion except it doesn't have an elementSize
  field.  This is the structure used to actually describe each region
  in the dump itself as the elementSize has no meaning in the dumped
  data itself.  Everything is dumped as bytes in little endian order.