Don't treat bitcode files specially in llvm-ar.

We really want bitcode files to behave as regular object files in archives, so
we don't need to track that a member is bitcode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185681 91177308-0d34-0410-b5e6-96231b3b80d8