Fix bug in ProcessPOSIX::IsAlive() exposed by r178324
- process in 'unloaded' state was (incorrectly) considered to be alive by POSIX plugin
- above caused a regression in TestProcessLaunch cases



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@178493 91177308-0d34-0410-b5e6-96231b3b80d8