un-deprecate AbsolutePath::appending(RelativePath) (#341)
un-deprecate AbsolutePath::appending(RelativePath) (#341)

motivation: concatinating rlative path to absolute path is a key API we should preserve it, but improve the implementation

chnages: remove deprecation marker from AbsolutePath::appending(RelativePath)

work to improve the implementation is tracked in https://github.com/apple/swift-tools-support-core/issues/336
apple/swift-tools-support-core,apple/swift-tools-support-core,apple/swift-tools-support-core,apple/swift-tools-support-core