Preliminary set of changes to ObjectFileMachO::ParseSymtab when lldb

is being run on iOS natively and we are examining a binary that is
in the shared-cache.  The shared cache may be set up to not load the
symbol names in memory (and may be missing some local symbols entirely,
to boot) so we need to read the on-disk-but-not-mapped-into-memory cache
of symbol names/symbols before we start processing the in-memory nlist
entries.  

This code needs to be reorganized into its own separate method, ideally
we'll find some way to not duplicate the nlist symbol handling.  But 
we need to handle this new format quickly and we'll clean up later.

Thanks for James McIlree for the patch.  Fixes <rdar://problem/11639018>.

llvm-svn: 158891