First steps towards automated releases / continuous deployment

[kinke]

Some convenience links:

AppVeyor:
* config: 
https://github.com/ldc-developers/llvm/blob/ldc-release_50/appveyor.yml
* log: https://ci.appveyor.com/project/kinke/llvm/build/1.0.18

CircleCI:
* config: 
https://github.com/ldc-developers/llvm/blob/ldc-release_50/.circleci/config.yml
* log: https://circleci.com/gh/ldc-developers/llvm/22