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

[Aaron Ballman via cfe-commits]

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@@ -123,6 +123,10 @@
 // CHECK-CHAR8_T_: "-fno-char8_t"
 
 
+// RUN: %clang_cl /dev/null /E -Xclang -dM 2> /dev/null | FileCheck -match-full-lines %s --check-prefix=STDCOFF
+// RUN: %clang_cl /dev/null /E -Xclang -dM /Zc:__STDC__ 2> /dev/null | FileCheck -match-full-lines %s --check-prefix=STDCON
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AaronBallman wrote:

I'd also like to see a test that ensures you can't *disable* the option. e.g., `-fno-ms-define-stdc` should be an error.

And this current test actually shows a bug -- this is a .cpp file, not a .c file; `/Zc:__STDC__` should do nothing/diagnose in C++ mode (https://godbolt.org/z/6W4oaKf9e); I'm on the fence as to whether `-fms-define-stdc` should be allowed in C++ mode or not (I'm inclined to say it should also be unsupported in C++ mode).

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