[NFC] Update FIXME for one VFS test

The VFS/subframework-symlink.m test is still XFAIL on Windows, but for
a different reason than those fixed in PR43272, so I've updated the
PR number.

clang/test/VFS/subframework-symlink.m

-// FIXME: PR43272
+// FIXME: PR44221
 // XFAIL: system-windows
 
 // Test that when a subframework is a symlink to another framework, we don't