[Windows] Introduce a switch for the `lldb-server` mode on Windows

Summary:
This patch introduces a switch, based on the environment variable
`LLDB_USE_LLDB_SERVER`, to determine whether to use the `ProcessWindows` plugin
(the old way) or the `lldb-server` way for debugging on Windows.

Reviewers: labath, amccarth, asmith, stella.stamenova

Reviewed By: labath, amccarth

Subscribers: mstorsjo, abidh, JDevlieghere, lldb-commits, leonid.mashinskiy

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68258

llvm-svn: 374325