First steps towards automated releases / continuous deployment

[kinke]

Our tailored LLVM now uses AppVeyor and CircleCI to build 
Win32/Win64/Linux-x86_64 release packages and upload them to a 
GitHub CI release: 
https://github.com/ldc-developers/llvm/releases/tag/CI

It could be extended to deploy builds with enabled assertions too 
(for the LDC CI tests) and further tweaked to upload tagged 
builds to the corresponding GitHub release etc.
I already requested macOS support from CircleCI.

As next steps, we can generalize the deployment of LDC CI builds 
(currently Windows only) to Linux/OSX/Windows and finally 
automate the creation of (the most popular) release packages on 
tagged builds.