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

[Aaron Ballman via cfe-commits]

================
@@ -2821,6 +2821,10 @@ 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]>,
+  HelpText<"Define __STDC__ in MSVC Compatibility mode">,
----------------
AaronBallman wrote:

```suggestion
  HelpText<"Define '__STDC__' to '1' in MSVC Compatibility mode">,
```

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