[clang][NFC] Mark CWG941 as implemented and add a test (#197202)
[CWG941](https://wg21.link/cwg941) allowed specializing deleted function
templates. Clang accepted this between 2.7 and 2.9, regressed and
started emitting redefinition errors between 3.0 and 3.8, then went back
to accepting in 3.9: https://godbolt.org/z/GKnf9je7j. I've marked it as
implemented since 3.9.