Change llvm-ar to use lib/Object.

This fixes two bugs is lib/Object that the use in llvm-ar found:
* In OS X created archives, the name can be padded with nulls. Strip them.
* In the constructor, remember the first non special member and use that in
  begin_children. This makes sure we skip all special members, not just the
  first one.

The change to llvm-ar itself consist of
* Using lib/Object for reading archives instead of ArchiveReader.cpp.
* Writing the modified archive directly, instead of creating an in memory
  representation.

The old Archive library was way more general than what is needed, as can
be seen by the diffstat of this patch.

Having llvm-ar using lib/Object now opens the way for creating regular symbol
tables for both native objects and bitcode files so that we can use those
archives for LTO.