[clang] Add /Zc:__STDC__ flag to clang-cl (#68690)

This commit adds the /Zc:\_\_STDC\_\_ argument from MSVC, which defines
\_\_STDC_\_.
This means, alongside stronger feature parity with MSVC, that things
that rely on \_\_STDC\_\_, such as autoconf, can work.
Link to MSVC documentation of this flag:
https://learn.microsoft.com/en-us/cpp/build/reference/zc-stdc?view=msvc-170