Add a test for devirtualization of virtual operator calls.

There was no test coverage for this before: Modifiying
EmitCXXOperatorMemberCallee() to not call CanDevirtualizeMemberFunctionCall()
didn't make any test fail.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223056 91177308-0d34-0410-b5e6-96231b3b80d8