README: document how to build swift-driver on Windows

[compnerd]

Although the toolchain distribution builds use CMake to build on Windows
(for bootstrapping the toolchain distribution), the development of
swift-driver on Windows requires building with swift-package-manager.
Update the documentation to explain how to build on Windows.

[compnerd]

@swift-ci please test

[cltnschlosser]

Awesome, I’ll have to try this out sometime. I gave up trying to build the toolchain on windows when I tried a few months back.

does swift test with those additional parameters, also work?

[compnerd]

Ah, alas I forgot that I never documented this in the README :-(. The swift driver bring up was all done with swift-package-manager (the only time to use CMake is when you are first building yourself a toolchain). Yes, swift-test works with the additional arguments ... in theory.

Currently, there have been some regressions (some of my own doing 😬). I would like to fix up the issues which I have caused with the recent rework of Path from swift-tools-support-core. There were a few other failures that I found due to the toolchain packaging having issues which I was unaware of. swiftlang/swift-installer-scripts#116 should help resolve the missing swift-api-digester which is required for the test suite. After that, there are ~14 failures due to path mismatches which I need to fix up to make swift-driver work with the latest swift-tools-support-core. Subsequently, there are 2 failures that I do not yet understand in SwiftDriverTests.testSupportedFeatureJson.

I would like to add that all of the 24 failures are regressions and that previously swift test with the documented flags would have given you a clean bill of health.

Hopefully @shahmishal will be able to help get CI enabled for Windows here.

Update: swiftlang/swift-installer-scripts#117 should resolve the 2 other failures. So, for now, just pin swift-tools-support-core at 58bb41c30c02ea4f46afdd31d90339ed0058c35f. With the other pending changes, you should even be able to run the test suite with swift test (given the documented arguments).