[lldb] Treat remote macOS debugging like any other remote darwin platform

Extract remote debugging logic from PlatformMacOSX and move it into
PlatformRemoteMacOSX so it can benefit from all the logic necessary for
remote debugging.

Until now, remote macOS debugging was treated almost identical to local
macOS debugging. By moving in into its own class, we can have it inherit
from PlatformRemoteDarwinDevice and all the functionality it provides,
such as looking at the correct DeviceSupport directory.

rdar://68167374

Differential revision: https://reviews.llvm.org/D92452