[Tests] First batch of cornercase tests for unordered atomics

Mixture of things we legally can't do, and things we're missing.  Once D57601 is in, the later will serve as a punch list.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353959 91177308-0d34-0410-b5e6-96231b3b80d8