[LCG] Re-implement the basic isParentOf, isAncestorOf, isChildOf, and

isDescendantOf methods on RefSCCs in terms of the forward edges rather
than the parent sets.

This is technically slower, but probably not interestingly slower, and
all of these routines were already so expensive that they're guarded
behind both !NDEBUG and EXPENSIVE_CHECKS.

This removes another non-critical usage of parent sets.

I've also added some comments to try and help clarify to any potential
users the costs of these routines. They're mostly useful for debugging,
asserts, or other queries.

llvm-svn: 310170