[clang] [clang-cl]: Add /std:c++23preview and update _MSVC_LANG for C++23 (PR #112378)

[via cfe-commits]

zmodem wrote:

With STL's test program from the thread on Discourse, this outputs:

```
C:\src\llvm-project>build\bin\clang-cl /EHsc /nologo /W4 /std:c++23preview meow.cpp && meow
Clang; __cplusplus: 202302; _MSVC_LANG: 202302; detected C++23
```

https://github.com/llvm/llvm-project/pull/112378