[CI] Add basic entry point script

[neonichu]

This should allow us to control the specifics of CI testing from within the repo as needed.

[compnerd]

This would be a problem for Windows. Can we use powershell instead?

[neonichu]

This would be a problem for Windows. Can we use powershell instead?

As far as I can tell, the current CI jobs on macOS/Linux are sh based, so I'm assuming Windows is anyway working differently on Swift CI?

[compnerd]

Yes, but those are running full toolchain builds. Doing so here would roughly equate to ~4h runs per PR.

[jakepetroules]

@compnerd Do you mean add a powershell script in addition to the sh script? Not instead of?

[compnerd]

@compnerd Do you mean add a powershell script in addition to the sh script? Not instead of?

Well, powershell is available on Linux, macOS, and windows and can therefore unify the CI script.

[dschaefer2]

Most of the toolchain build is in Python, no?

[neonichu]

Most of the toolchain build is in Python, no?

Not used by the Windows build, AFAIK.

unify the CI script.

I think we should worry about unification once it is possible.

Right now, there's no Windows CI job at all on this repository and even once we add it, it'll have to use CMake in order to match the rest of the Windows build.

The purpose of this PR was just to pull a part of the existing CI jobs into the repo, nothing more. If we don't end up wanting that, I can close it, but I am not aiming for more right now.

[jakepetroules]

@swift-ci please test macOS