Windows: Create a dangling junction when symlink target doesn't exist

On Unix system, `ln -s foo bar` will create an dangling symlink bar -> foo
even foo doesn't exist. The current implementation of createSymbolicLink for
Windows fails in this situtation. And this lead to #2474.

In this change, we create a dangling junction when the target doesn't exist
which mimics the behavior on Unix.

Fixed #2474

Change-Id: I442ca3e2fb20b76c9b5bbfee903299fe51481f43
PiperOrigin-RevId: 157694631

src/main/java/com/google/devtools/build/lib/windows/WindowsFileSystem.java

@@ -347,7 +347,8 @@ protected void createSymbolicLink(Path linkPath, PathFragment targetFragment) th
     try {
       java.nio.file.Path link = getIoFile(linkPath).toPath();
       java.nio.file.Path target = getIoFile(targetPath).toPath();
-      if (target.toFile().isDirectory()) {
+      // Still Create a dangling junction if the target doesn't exist.
+      if (!target.toFile().exists() || target.toFile().isDirectory()) {
         WindowsFileOperations.createJunction(link.toString(), target.toString());
       } else {
         Files.copy(target, link);