[ADT] Add SmallVector::pop_back_n

Adds a method called pop_back_n to SmallVector.
This is more readable and less error prone than the alternatives of using
```lang=c++
Vector.resize(Vector.size() - N);
Vector.erase(Vector.end() - N, Vector.end());
for (unsigned I = 0;I<N;++I) Vector.pop_back();
```

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D90576