See https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-180

One recent user reported case was #14310

• /Zc:__STDC__ isn't using --ms_stdc and defining __STDC__.
• /Za isn't defing __STDC__.
  ...it seems like there could be a lot more...maybe we could postpone this till users ask for specific args to be handled? Some of these are probably pretty rarely used?