[Windows] Build SwiftCompilerPlugin module when testing swift

[rintaro]

build.ps1 doesn't test executable plugins at all. That means it doesn't even try to compile SwiftComplerPlugin module. At least, let's build it when testing Swift so we can verify the build breakage in PR testing.

[rintaro]

@swift-ci Please test Windows

[rintaro]

@swift-ci Please smoke test macOS

[rintaro]

@swift-ci Please smoke test Linux

[compnerd]

I'm fine with this, but why is this not wired up into the check-swift target if its meant for testing?

[rintaro]

why is this not wired up into the check-swift target if its meant for testing?

As long as it imports Foundation, it cannot be a part of swift-test 😢
For non-Windows, we build swift-syntax/Examples (using swiftpm) which includes macros using SwiftCompilerPlugin as a part of swift-syntax tests