Fixes #3571 for <filesystem> and <experimental/filesystem>.

The fix for <experimental/filesystem> affects the redist because it changes the implementation of _Equivalent in filesys.cpp that is exported by msvcp140.dll.

There is no additional test coverage because a reliable test for this issue would involve setting up a drive commonly producing such transient file IDs. But I verified manually with a drive shared by Remote Desktop that (a) false positives by std::filesystem::equivalent and std::experimental::filesystem::equivalent are present without this PR and (b) are gone with it. I also added comments at the top of _Equivalent and __std_fs_equivalent that reference #3571 and serve as a reminder why the file handles can't be closed before file IDs for both files have been obtained.