TarWriter: Use Ustar header's "prefix" field to store long filenames.
TarWriter: Use Ustar header's "prefix" field to store long filenames.

Tar's Ustar header has the "prefix" field to store a directory
part of a filename. It is not as flexible as the PAX-extended
filename because there's still a limitation on the maximum filename
size, but it mitigates the situation.

This patch should unbreak some Windows buildbots that uses very
old tar command.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@291340 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm