Fix Path Normalization on Windows
Fix Path Normalization on Windows

Absolute paths on Windows do not begin with a leading slash. Defer to
llvm's is_absolute instead which handles cross platform paths.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild