Move common functionality from processwindows into processdebugger

Summary:
This change extracts functionalities from processwindows into a
introduced processdebugger that can be reused in native process
debugging. 

The main reason is that the native process debugging
can't directly be based on processwindows or be implemented
as a pass-through to this plugin since the plugin has ties to
Target and Process classes that are needed in host debugging but
not necessary in native debugging.

Reviewers: labath, Hui, jfb, clayborg, amccarth

Reviewed By: labath

Subscribers: amccarth, dexonsmith, mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D63166

llvm-svn: 364210