Changed profiling method for ImmutableMap to once again just use its

unique ImutAVLTree* for profiling.

Modified ImutAVLTree:
 (1) changed ComputeHash() to ComputeDigest() and
 (2) changed Profile() to use the computed digest and
 (3) modified insertion of IMutAVLTree into the FoldingSet owned by
     the ImutAVLTreeFactory object to use profiling instead of computing
     a direct hash.  This fixes a bug where our abuse of the FoldingSet would
     not work when the FoldingSet was resized.

llvm-svn: 46753