Introduce `SendableOpaquePointer` #16

Job	Run time
Unit tests / Unit tests (swift:5.9-jammy, 5.9, true)	5m 9s
Unit tests / Unit tests (swift:5.10-jammy, 5.10, true)	3m 24s
Unit tests / Unit tests (swift:6.0-jammy, 6.0, true)	5m 7s
Unit tests / Unit tests (swiftlang/swift:nightly-6.0-jammy, nightly-6.0, true)	5m 52s
Unit tests / Unit tests (swiftlang/swift:nightly-main-jammy, nightly-main, true)	5m 48s
Cxx interop / Windows (6.0)	3s
Cxx interop / Linux (5.9)	2m 17s
Cxx interop / Windows (nightly-6.0)	1s
Cxx interop / Linux (5.10)	2m 5s
Cxx interop / Windows (nightly-main)	1s
Cxx interop / Linux (6.0)	2m 33s
Soundness / API breakage check	0s
Soundness / Broken symlinks check	9s
Cxx interop / Linux (nightly-6.0)	2m 50s
Cxx interop / Linux (nightly-main)	2m 52s
Soundness / Documentation check	0s
Soundness / Format check	59s
Soundness / License headers check	6s
Soundness / Python lint check	6s
Soundness / Shell check	50s
Soundness / Unacceptable language check	7s
Soundness / YAML lint check	6s
	40m 25s

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