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

[Aaron Ballman via cfe-commits]

================
@@ -389,6 +389,7 @@ SYMBOL(cosh, None, <math.h>)
 SYMBOL(coshf, None, <math.h>)
 SYMBOL(coshl, None, <math.h>)
 SYMBOL(cosl, None, <math.h>)
+SYMBOL(countof, None, <stdcountof.h>)
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AaronBallman wrote:

Yup, I missed:
```
// Generated from cppreference offline HTML book (modified on 2018-10-28).
//===----------------------------------------------------------------------===//
```
at the top of the file. So I'll pull this change back, good catch!

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