Don't depend on a llvm-ar specific behavior.

GNU ar when not given the a or b modifiers replaces archive members in the
same location of the old ones. I am about to implement that in llvm-ar. For
now, just don't depend on the current llvm-ar behavior on this test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186823 91177308-0d34-0410-b5e6-96231b3b80d8