Bazel client: implement PathExists on Windows

Checking if a path exists is surprisingly hard on
Windows. The most convenient API functions are
PathFileExists and GetFileAttributes but neither
of them follows junctions. To check if a junction
is dangling, we have to resolve it all the way.

This change adds a JunctionResolver class to
file_windows, which can resolve junctions (if they
aren't dangling) and non-junctions (in this case
just checks their existence).

See https://github.com/bazelbuild/bazel/issues/2107
See https://github.com/bazelbuild/bazel/issues/2181

--
PiperOrigin-RevId: 143645274
MOS_MIGRATED_REVID=143645274

src/main/cpp/util/file_platform.h

diff --git a/src/main/cpp/util/file_platform.h b/src/main/cpp/util/file_platform.h
index 5354052..b0458e8 100644
--- a/src/main/cpp/util/file_platform.h
+++ b/src/main/cpp/util/file_platform.h
@@ -44,7 +44,7 @@
 // Returns true on success. In case of failure sets errno.
 bool UnlinkPath(const std::string &file_path);
 
-// Returns true if this path exists.
+// Returns true if this path exists, following symlinks.
 bool PathExists(const std::string& path);
 
 // Returns the real, absolute path corresponding to `path`.