[debugserver] Add one additional sleep before attaching after waiting

It's possible for debugserver to attach to a process during the handoff
between /usr/lib/dyld and the dyld in the shared cache. When that
happens, we may end up in a state where there is no dyld in the process
and our debugging session is doomed. To make that scenario a lot less
likely, we can insert a sleep right before attaching after waiting to
find the right pid.

rdar://105513180

Differential Revision: https://reviews.llvm.org/D144311