Introduce `SendableOpaquePointer` #17

Job	Run time
Unit tests / Unit tests (swift:5.9-jammy, 5.9, true)	4m 46s
Unit tests / Unit tests (swift:5.10-jammy, 5.10, true)	4m 30s
Unit tests / Unit tests (swift:6.0-jammy, 6.0, true)	4m 58s
Unit tests / Unit tests (swiftlang/swift:nightly-6.0-jammy, nightly-6.0, true)	4m 0s
Unit tests / Unit tests (swiftlang/swift:nightly-main-jammy, nightly-main, true)	5m 26s
Cxx interop / Linux (5.9)	2m 4s
Cxx interop / Linux (5.10)	2m 3s
Cxx interop / Linux (6.0)	2m 25s
Cxx interop / Linux (nightly-6.0)	2m 53s
Cxx interop / Linux (nightly-main)	2m 57s
Cxx interop / Windows (6.0)	3s
Soundness / API breakage check	0s
Soundness / Broken symlinks check	5s
Cxx interop / Windows (nightly-6.0)	1s
Cxx interop / Windows (nightly-main)	2s
Soundness / Documentation check	0s
Soundness / Format check	42s
Soundness / License headers check	6s
Soundness / Python lint check	7s
Soundness / Shell check	43s
Soundness / YAML lint check	7s
Soundness / Unacceptable language check	8s
	38m 6s

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