most of the behavior we want, but wrap the predicate in one which erases
elements from the set if they pass the predicate. Oh what I wouldn't
give for a lambda here.
Let me know if the predicate wrapping is too much magic. ;]
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165076 91177308-0d34-0410-b5e6-96231b3b80d8
