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

[Max Winkler via cfe-commits]

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@@ -291,6 +291,7 @@ LANGOPT(HIPStdParInterposeAlloc, 1, 0, "Replace allocations / deallocations with
 
 LANGOPT(OpenACC           , 1, 0, "OpenACC Enabled")
 
+LANGOPT(MSVCEnableStdcMacro , 1, 0, "Define __STDC__ with '-fms-compatability'")
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MaxEW707 wrote:

```suggestion
LANGOPT(MSVCEnableStdcMacro , 1, 0, "Define __STDC__ with '-fms-compatibility'")
```

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