Implement CXCursor.IsExternC for VarDecls. #568

Job	Run time
linux-x64 (x64, debug, ubuntu)	3m 48s
macos-arm64 (arm64, debug, macos)	4m 11s
linux-arm64 (arm64, release, ubuntu)	3m 34s
linux-arm64 (arm64, debug, ubuntu)	3m 34s
macos-arm64 (arm64, release, macos)	3m 40s
windows-arm64 (arm64, release, windows)	6m 6s
linux-x64 (x64, release, ubuntu)	3m 22s
windows-arm64 (arm64, debug, windows)	5m 53s
windows-x64 (x64, debug, windows)	5m 27s
windows-x64 (x64, release, windows)	5m 8s
publish-nightlies-github	-1s
publish-nightlies-azure	-1s
sign-nuget	-1s
	44m 40s

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