Fix being able to get a thread result when calling HostThreadPosix::Join(). It...

Fix being able to get a thread result when calling HostThreadPosix::Join(). It was broken when initially checked in by getting the thread result into a temporary variable and never doing anything with it. Most threads in LLDB don't look at their thread results, but launching processes in a terminal window on MacOSX does require getting a thread result and this broke "process launch --tty" on darwin.

<rdar://problem/19308966>

llvm-svn: 225224