Reverted implementation of ImmutableMap::find() to return a TreeTy* instead of

an iterator, since the implementation returned an iterator that pointed to a
different node! Renamed this implementation to SlimFind() so that users do not
expect it to return an iterator (it is a more efficient implementation than
returning an iterator if the user just wants to find the value of a key).

Added a FIXME to implement ImmutableMap::find() that returns an iterator.

llvm-svn: 46150