Replaced (FoldingSet) profiling of ImutAVLTree with a hashing based scheme. The

Replaced (FoldingSet) profiling of ImutAVLTree with a hashing based scheme. The
problem was that we previously hashed based on the pointers of the left and
right children, but this is bogus: we can easily have different trees that
represent the same set. Now we use a hashing based scheme that compares the
*contents* of the trees, but not without having to do a full scan of a tree. The
only caveat is that with hashing is that we may have collisions, which result in
two different trees being falsely labeled as equivalent. If this becomes a
problem, we can add extra data to the profile to hopefully resolve most
collisions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46224 91177308-0d34-0410-b5e6-96231b3b80d8