Revert "More fixes to get good error messages for bad archives."

This reverts commit r277540. It breaks the build with:

../lib/Object/Archive.cpp:264:41: error: return type of out-of-line definition of 'llvm::object::ArchiveMemberHeader::getUID' differs from that in the declaration
Expected<unsigned> ArchiveMemberHeader::getUID() const {
~~~~~~~~~~~~~~~~~~                      ^
include/llvm/Object/Archive.h:53:12: note: previous declaration is here
  unsigned getUID() const;
  ~~~~~~~~ ^

llvm-svn: 277627