Add llvm::enumerate() to stl extras

[Zachary Turner via llvm-commits]

I frequently find myself resorting to index based enumeration because
ranged based fors do not allow you to access the index.  With
llvm::enumerate(), you can write something like this:

for (auto X : enumerate(my_container)) {
  if (X.Index == 1)
    continue;
  printf("%d", X.Value);
}

It handles both const and non-const containers, so that for non-const
containers you can modify the underlying sequence item (e.g. the `Value`
member of the returned enumerator is a reference).

Phab is down for now so I'm using a regular email, but if this is too
inconvenient i can wait for Phab to come back up and resubmit.
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