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

[Erich Keane via cfe-commits]

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@@ -252,8 +252,8 @@ static bool warnByDefaultOnWrongCase(StringRef Include) {
     .Cases("assert.h", "complex.h", "ctype.h", "errno.h", "fenv.h", true)
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erichkeane wrote:

The fact that these aren't tablegened somewhere is frightening!

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