[concurrency] conform Deque.Iterator to unchecked-Sendable

[glessard]

On a toolchain that contains swiftlang/swift#75782 (79bf5d51ca4b55afdb29102f411898d78cbe0bf2), Deque no longer compiles in Swift 6 mode.

extension Deque._Storage: @unchecked Sendable {} makes it compile, though it may not be the best solution?

Addresses rdar://134300306

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.

[glessard]

@swift-ci please test

[glessard]

@swift-ci please test

[glessard]

@swift-ci please test

[lorentey]

The source file removal needs to be reflected in CMakeLists.txt and Xcode/Collections.xcodeproj

[glessard]

@swift-ci please test