Fix Sendable warnings when Swift 6 mode is enabled

[mergesort]

Warnings begin to appear about numerous types being shared across actor boundaries even though they're not marked as Sendable when you enable these flags.

swiftSettings: .unsafeFlags(["-Xfrontend", "-warn-concurrency", "-Xfrontend", "-enable-actor-data-race-checks"])

These warnings aren't time-critical to fix but will need to be addressed in the future.