[Symbolize] Improve the ownership of parsed objects.

This code changes the way Symbolize handles parsed binaries: now
parsed OwningBinary<Binary> is not broken into (binary, memory buffer)
pair, and is just stored as-is in a cache. ObjectFile components
of Mach-O universal binaries are also stored explicitly in a
separate cache.

Additionally, this change:
* simplifies the code that parses/caches binaries: it's now done
  in a single place, not three different functions.
* makes flush() method behave as expected, and actually clear
  the cached parsed binaries and objects.
* fixes a dangling pointer issue described in
  http://reviews.llvm.org/D15638

llvm-svn: 256041