Add ObjCMethodDecl.Selector. #537

Job	Run time
windows-arm64 (arm64, debug, windows)	6m 1s
linux-arm64 (arm64, release, ubuntu)	3m 34s
linux-x64 (x64, release, ubuntu)	3m 38s
linux-arm64 (arm64, debug, ubuntu)	3m 31s
macos-arm64 (arm64, release, macos)	3m 40s
macos-arm64 (arm64, debug, macos)	3m 39s
windows-x64 (x64, debug, windows)	5m 48s
linux-x64 (x64, debug, ubuntu)	3m 41s
windows-arm64 (arm64, release, windows)	5m 50s
windows-x64 (x64, release, windows)	5m 24s
publish-nightlies-azure	-1s
publish-nightlies-github	-1s
sign-nuget	-1s
	44m 43s

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