Use a bool for whether or not an abbreviation has children rather than
using a full uint16_t with the flag value... which happens to be
0 or 1. Update the class for bool values and rename functions slightly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202921 91177308-0d34-0410-b5e6-96231b3b80d8