[Windows] Special case ~ as an absolute path
[Windows] Special case ~ as an absolute path

PathIsRelativeW doesn't consider ~ to be an absolute path, so we special
case it in to match posix behaviour

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation