Object: Replace NewArchiveIterator with a simpler NewArchiveMember class. NFCI.

The NewArchiveIterator class has a problem: it requires too much context. Any
memory buffers added to the archive must be stored within an Archive::Member,
which must have an associated Archive. This makes it harder than necessary
to create new archive members (or new archives entirely) from scratch using
memory buffers.

This patch replaces NewArchiveIterator with a NewArchiveMember class that
stores just the memory buffer and the information that goes into the archive
member header.

Differential Revision: http://reviews.llvm.org/D21721

llvm-svn: 274183