[clang] [Clang] Add __builtin_counted_by_ref builtin (PR #114495)

[Bill Wendling via cfe-commits]

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@@ -16,6 +16,8 @@ void test1(struct fam_struct *ptr, int size, int idx) {
 
   *__builtin_counted_by_ref(ptr->array) = size;             // ok
   *__builtin_counted_by_ref(&ptr->array[idx]) = size;       // ok
+  *__builtin_counted_by_ref(&ptr->array) = size;            // ok
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bwendling wrote:

`*__builtin_counted_by_ref(ptr->array[idx]) = size;` doesn't work because the builtin requires a pointer to the FAM. (If you meant `&ptr->array[idx]`, that does work.)

I don't have any use case for `&ptr->array` or the like. I just don't want to be overly restrictive on what's considered a valid argument to the builtin. I think I'll do a comparison with gcc's version and see what they accept. We can match them.

https://github.com/llvm/llvm-project/pull/114495