Add element-atomic mem intrinsic canary tests for Memory Sanitizer.

Add element-atomic mem intrinsic canary tests for Memory Sanitizer.

Summary:
Add canary tests to verify that MSAN currently does nothing with the element atomic memory intrinsics for memcpy, memmove, and memset.

Placeholder tests that will fail once element atomic @llvm.mem[cpy|move|set] instrinsics have been added to the MemIntrinsic class hierarchy. These will act as a reminder to verify that MSAN handles these intrinsics properly once they have been added to that class hierarchy.

Reviewers: reames

Reviewed By: reames

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35510

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308251 91177308-0d34-0410-b5e6-96231b3b80d8