[ADT] Add an 'llvm::seq' function which produces an iterator range over

a sequence of values.

It increments through the values in the half-open range: [Begin, End),
producing those values when indirecting the iterator. It should support
integers, iterators, and any other type providing these basic arithmetic
operations.

This came up in the C++ standards committee meeting, and it seemed like
a useful construct that LLVM might want as well, and I wanted to
understand how easily we could solve it. I suspect this can be used to
write simpler counting loops even in LLVM along the lines of:

  for (int i : seq(0, v.size())) {
    ...
  };

As part of this, I had to fix the lack of a proxy object returned from
the operator[] in our iterator facade.

Differential Revision: http://reviews.llvm.org/D17870

llvm-svn: 269390