[dsymutil] Implement the BinaryHolder object and gain archive support.

This object is meant to own the ObjectFiles and their underlying
MemoryBuffer. It is basically the equivalent of an OwningBinary
except that it efficiently handles Archives. It is optimized for
efficiently providing mappings of members of the same archive when
they are opened successively (which is standard in Darwin debug
maps, objects from the same archive will be contiguous).

Of course, the BinaryHolder will also be used by the DWARF linker
once it is commited, but for now only the debug map parser uses it.

With this change, you can run llvm-dsymutil on your Darwin debug build
of clang and get a complete debug map for it.

Differential Revision: http://reviews.llvm.org/D6690

llvm-svn: 225207