[lldb] [llgs] Support owning and detaching extra processes

Add a NativeDelegate API to pass new processes (forks) to LLGS,
and support detaching them via the 'D' packet.  A 'D' packet without
a specific PID detaches all processes, otherwise it detaches either
the specified subprocess or the main process, depending on the passed
PID.

Differential Revision: https://reviews.llvm.org/D100191