[lldb] Simplify HostThreadMacOSX

The class is using an incredibly elaborate setup to create and destroy
an NSAutoreleasePool object. We can do it in a much simpler way by
making those calls inside our thread startup function.

The only effect of this patch is that the pool gets released at the end
of the ThreadCreateTrampoline function, instead of slightly later, when
pthreads begin thread-specific cleanup. However, the key destruction
order is unspecified, so nothing should be relying on that.

I didn't find a specific reason for why this would have to be done that
way in git history. It seems that before D5198, this was thread-specific
keys were the only way an os implementation (in Host::ThreadCreated)
could attach some value to a thread.

Differential Revision: https://reviews.llvm.org/D120322