[lldb] [gdb-remote server] Introduce new stop reasons for fork and vfork

Introduce three new stop reasons for fork, vfork and vforkdone events.
This includes server support for serializing fork/vfork events into
gdb-remote protocol.  The stop infos for the two base events take a pair
of PID and TID for the newly forked process.

Differential Revision: https://reviews.llvm.org/D100196