Make MissingInitializer conform to Sendable #679

Job	Run time
Lint Swift	1m 16s
Check Markdown links	20s
Build and Test on Linux	6m 25s
Build and Test on Xcode 15	3m 42s
Build Package Integration on Xcode 15	1m 30s
Build Multi Project Integration on Xcode 15	4m 17s
Build Project Integration on Xcode 15	2m 21s
Build with xcodebuild on Xcode 15 (generic/platform=ios)	5m 15s
Build with xcodebuild on Xcode 15 (platform=macos)	3m 35s
Build with xcodebuild on Xcode 15 (generic/platform=tvos)	5m 0s
Build with xcodebuild on Xcode 15 (generic/platform=watchos)	9m 35s
Build with xcodebuild on Xcode 15 (generic/platform=visionos)	4m 21s
	47m 37s

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