[SetVector] Add erase() method

This is a recommit of r264414 after fixing the buildbot failure caused by
incompatible use of std::vector.erase().

The original message:

Add erase() which returns an iterator pointing to the next element after the
erased one. This makes it possible to erase selected elements while iterating
over the SetVector :
  while (I != E)
    if (test(*I))
      I = SetVector.erase(I);
    else
      ++I;

Reviewers: qcolombet, mcrosier, MatzeB, dblaikie

Subscribers: dberlin, dblaikie, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D18281

llvm-svn: 264450