Modified node creation of ImutAVLTree to do a hash lookup for an existing

node in the FoldingSet of nodes held by the Factory object.  If we
we find a node with a matching hash, we do a full structural comparison.
Nodes are also now inserted into the FoldingSet only when we mark them
Immutable, as their children can change during intermediate-rebalancing.

The 'Profile' method for ImutAVLTree is no longer used when looking up
existing ImutAVLTrees with a given set of contents; instead the Profile method
is used by other clients that wish to insert such a tree into a folding set.
This means that we are not using FoldingSet in ImutAVLTreeFactory in the way
it was intended, but instead are using it as an opaque hashtable.

llvm-svn: 46717