Avoid hang in attach-by-name test case
- Check that process attach succeeded before attempting to WaitForProcessToStop (observed to cause hangs on Linux)
- Update comment in TestHelloWorld case -- attaching by name still broken



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@178491 91177308-0d34-0410-b5e6-96231b3b80d8