Recent updates to azure include the VS2019 hosted builder. Since I have been building with VS2019 locally, I have decided to setup additional CI for Windows We now have VS2017 and VS2019 builds for x64 on azure. I also have added a badge for it on https://github.com/compnerd/windows-swift.
I think that we should keep both 2017 and 2019 working. At this point, I don't think that it is viable to keep in line with the LLVM restrictions - I think that just jumping to 2017 rather than keeping to 2015 for the minimum isn't too bad. Azure doesn't even have 2015 as an option, so even if we were to get it to build with 2015, keeping it building is difficult. Unless someone wants to do the work to get VS2015 building and donate resources to have CI for that, I think that we should just claim that the minimum requirement is VS2017.
One thing which I wonder about is if there is any reason to invest in a 32-bit toolchain. I think that a 32-bit runtime is still interesting, but building on 32-bit is challenging as program sizes balloon. In fact, due to debug information, even building a 32-bit toolchain requires a cross-compilation using a 64-bit toolchain (well, I believe that it is possible to build using a 32-bit MSVC toolchain on Windows).