[clang] [C2y] Add stdcountof.h (PR #140890)

[Aaron Ballman via cfe-commits]

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@@ -49,6 +49,11 @@ module cstd [system] [no_undeclared_includes] {
     export *
   }
 
+  module stdcountof {
+    header "stdcountof.h"
+    export *
+  }
+
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AaronBallman wrote:

Good catch! I've updated that test as well.

https://github.com/llvm/llvm-project/pull/140890