Enable API breakage check when SwiftPM no longer diagnoses SPI changes as breaking

[ahoppen]

We don’t consider SPI part of swift-syntax’s public API and can break it as much as we want. But swift package diagnose-api-breaking-changes reports them as breaking changes (swiftlang/swift#79990). This generates too much noise for the API breakage check to be useful for swift-syntax.

[ahoppen]

Synced to Apple’s issue tracker as rdar://146703651