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.
I've made it in the way similar to Zachary have made for the SymbolFileNativePDB plugin. An environment variable could be more convenient e.g. to run a bunch of tests using the lldb-test option.
The environment variable for using the native PDB was always intended to be temporary, since the native PDB reader would eventually be the only PDB reader.
In general, I find environment variables harder to manage (especially on Windows) than command line options. They're invisible global variables that compound all the environmental issues that make building and testing so flaky and hard to diagnose.