PSA: Changes to embedding Python Windows incoming

If you don’t build LLDB on Windows, you can stop reading now.

I’ve just uploaded for review, which is going to change the way people build on Windows (for the better).

I plan to commit it later today, but I wanted to give people a heads up in case they don’t closely follow the commits list that this will be incoming.

The TL;DR of the review description is that after this patch:
a) You will only need to specify 1 CMake variable instead of 3
b) You can clear your PYTHONPATH and PYTHONHOME variables and everything will still work for every combination of {Debug,Release} x {x64,x86}