ProcessLaunchInfo: Remove Target reference
The target was being used in FinalizeFileActions to provide default
values for stdin/out/err. Also, most of the logic of this function was
very specific to how the lldb's Target class wants to launch processes,
so I, move it to Target::FinalizeFileActions, inverting the dependency.
The only piece of logic that was useful elsewhere (lldb-server) was the
part which sets up a pty and relevant file actions. I've kept this part
This makes ProcessLaunchInfo independent of any high-level lldb constructs.
Reviewers: zturner, jingham, teemperor
Differential Revision: https://reviews.llvm.org/D56196