[NFC] rename member of BitTestBlock and JumpTableHeader

Follow up to suggestions in D109103 via hans:
  I think UnreachableDefault (or UnreachableFallthrough) would be a
  better name now, since it doesn't just omit the range check, it also
  omits the last bit test.

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D109455