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

[Max Winkler via cfe-commits]

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@@ -2934,6 +2934,10 @@ def fms_compatibility : Flag<["-"], "fms-compatibility">, Group<f_Group>,
   Visibility<[ClangOption, CC1Option, CLOption]>,
   HelpText<"Enable full Microsoft Visual C++ compatibility">,
   MarshallingInfoFlag<LangOpts<"MSVCCompat">>;
+def fms_define_stdc : Flag<["-"], "fms-define-stdc">, Group<f_Group>,
+  Visibility<[ClangOption, CC1Option]>,
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MaxEW707 wrote:

```suggestion
  Visibility<[ClangOption, CC1Option, CLOption]>,
```

In my opinion similar to `-fms-compatibility` and `-fms-extensions` we should allow users to do `-fms-define-stdc` when in the cl driver mode.

https://github.com/llvm/llvm-project/pull/68690