Change ObjectFileMachO::ParseSymtab to read the external
function stub routine addresses from an in-memory-only
MachO object file.  This was the only remaining part of
ParseSymtab() that was assuming a file exists.
<rdar://problem/13139585> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@174455 91177308-0d34-0410-b5e6-96231b3b80d8