[LCG] Completely remove the parent set and leaf tracking for RefSCCs.

[LCG] Completely remove the parent set and leaf tracking for RefSCCs.

After the previous series of patches, this is now trivial and deletes
a pretty astonishing amount of complexity. This has been a long time
coming, as the move toward a PO sequence of RefSCCs started eroding the
underlying use cases for this half of the data structure.

Among the biggest advantages here is that now there aren't two
independent data structures that need to stay in sync.

Some of my profiling has also indicated that updating the parent sets
was among the most expensive parts of the lazy call graph. Eliminating
it whole sale is likely to be a nice win in terms of compile time.

Last but not least, I had discussed with some folks previously keeping
it around for asserts and other correctness checking, but once the
fundamentals of the parent and child checking were implemented without
the parent sets their value in correctness checking was tiny and no
where near worth the cost of the complexity required to keep everything
up-to-date.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310171 91177308-0d34-0410-b5e6-96231b3b80d8