Support using the extended thread-id syntax with Hg packet to select
a subprocess. This makes it possible to start providing support for
running some of the debugger packets against another subprocesses.
Differential Revision: https://reviews.llvm.org/D100261
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