Start adding thin archive support.

This is just sufficient for 'ar t' to work.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224307 91177308-0d34-0410-b5e6-96231b3b80d8
Remove unused member and simplify. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224309 91177308-0d34-0410-b5e6-96231b3b80d8