Merging r281287:

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r281287 | rtrieu | 2016-09-12 18:37:01 -0700 (Mon, 12 Sep 2016) | 2 lines

Handle empty message in static_asserts.

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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_39@286918 91177308-0d34-0410-b5e6-96231b3b80d8