Merging r196768:

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r196768 | majnemer | 2013-12-09 01:04:00 -0800 (Mon, 09 Dec 2013) | 5 lines

ADT: Implement MutableArrayRef::reverse_iterator

This adds rbegin/rend methods to MutableArrayRef, they will be used by a
follow-on commit in clang.

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llvm-svn: 196945