Switch the SetVector::remove_if implementation to use partition which

preserves the values of the relocated entries, unlikely remove_if. This
allows walking them and erasing them.

Also flesh out the predicate we are using for this to support the
various constraints actually imposed on a UnaryPredicate -- without this
we can't compose it with std::not1.

Thanks to Sean Silva for the review here and noticing the issue with
std::remove_if.

llvm-svn: 165073