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

[Max Winkler via cfe-commits]

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@@ -8065,6 +8069,9 @@ void Clang::AddClangCLArgs(const ArgList &Args, types::ID InputType,
    CmdArgs.push_back("-fno-wchar");
  }
 
+ if (!types::isCXX(InputType) && Args.hasArg(options::OPT_fms_define_stdc))
+   CmdArgs.push_back("-fms-define-stdc");
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MaxEW707 wrote:

This can be deleted. Since `/Zc:__STDC__` is aliased to `fms-define-stdc` this is already handled in the code above.
`AddClangCLArgs` is intended to handle cl.exe options that are not aliased.

If you want `-fms-define-stdc` to be usable when in CL mode then you can add `CLOption` to the visibility as I mentioned above in `Options.td`.

My preference is to remove this code and just add `CLOption`.

See `Option::accept` inside `Option.cpp` for where aliased args are turned into the argument they alias.

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