ProcessLaunchInfo: remove Debugger reference
The Debuffer object was being used in "GetListenerForProcess" to provide
a default listener object if one was not specified in the launch_info
Since all the callers of this function immediately passed the result to
Target::CreateProcess, it was easy to move this logic there instead.
This brings us one step closer towards being able to move the LaunchInfo
classes to the Host layer (which is there the launching code that
consumes them lives).
Reviewers: zturner, jingham, teemperor
Differential Revision: https://reviews.llvm.org/D56174