[lldb] Make `process connect` blocking in synchronous mode.

In synchronous mode, the process connect command and its aliases should
wait for the stop event before claiming the command is complete.
Currently, the stop event is always handled asynchronously by the
debugger.

The implementation takes the same approach as Process::ResumeSynchronous
which hijacks the event and handles it on the current thread. Similarly,
after this patch, the stop event is part of the command return object,
which is the property used by the test case.

Differential revision: https://reviews.llvm.org/D83728