combase: implement WindowsSubstring

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
https://testbot.winehq.org/JobDetails.pl?Key=12173

Your paranoid android.


=== w8 (32 bit string) ===
string.c:258: Test failed: Duplicated string created new string