This ensures that the last child indexes are calculated in linear time and
can later be queried in constant time by getLastChild.
The baseline situation was that individual calls to getLastChild were linear in the
size of DieArray. Calling getLastChild once for every DWARFDebugInfoEntry was
amortized quadratic in the size of DieArray.
Running "llvm-gsymutil --convert llvm-gsymutil --quiet" using a RelWithDebInfo build
of llvm-gsymutil (after recent other optimizations) and gathering a profile using
perf showed that llvm::DWARFUnit::getLastChild is the no. 1 function, accounting
for 9.9% of the CPU time, and llvm::DWARFUnit::getSibiling is the no. 4 function,
accounting for 4.76% of the CPU time.