Merging r314733:

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r314733 | rsmith | 2017-10-02 15:43:36 -0700 (Mon, 02 Oct 2017) | 5 lines

PR33839: Fix -Wunused handling for structured binding declarations.

We warn about a structured binding declaration being unused only if none of its
bindings are used.

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llvm-svn: 319847