Remove declare but not implemented methods.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185904 91177308-0d34-0410-b5e6-96231b3b80d8
Compute the size of an archive member in the constructor.

It is always computed the same way (by parsing the header). Doing it in the
constructor simplifies the callers a bit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185905 91177308-0d34-0410-b5e6-96231b3b80d8