Clean up handling of FreeBSD thread list on Launch / Attach

Instead of directly manipulating the thread list in Launch and Attach,
just rely on RefreshStateAfterStop to populate the initial list.

Review: http://llvm-reviews.chandlerc.com/D1565


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@189889 91177308-0d34-0410-b5e6-96231b3b80d8