[lldb] [client] Implement follow-fork-mode

Implement a new target.process.follow-fork-mode setting to control
LLDB's behavior on fork.  If set to 'parent', the forked child is
detached and parent continues being traced.  If set to 'child',
the parent is detached and child becomes traced instead.

Differential Revision: https://reviews.llvm.org/D100503