Replace archive members in the old position.

This matches gnu archive behavior and since archive member order can change
which member is used, not changing the order on replacement looks like the
right thing to do.

This patch also refactors the logic for which archive member to keep and
whether to move it to a helper function (computeInsertAction). The
nesting in computeNewArchiveMembers was getting a bit confusing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186829 91177308-0d34-0410-b5e6-96231b3b80d8