[clang] [clang-cl] Support /std:clatest (PR #147284)

[Hans Wennborg via cfe-commits]

https://github.com/zmodem approved this pull request.

Thanks!

(For my curiosity: seems the flag was added to the MSVC docs in https://github.com/MicrosoftDocs/cpp-docs/commit/0b7e40a3c5f689bcf27ec7bcba8758b5f0e39a28)

https://github.com/llvm/llvm-project/pull/147284