[clang] [BoundsSafety][Sema] Disallow using a bitfield with __builtin_counted_by_ref() (PR #231591)

[Dan Liew via cfe-commits]

================
@@ -185,3 +185,12 @@ struct ptr_no_attr {
 void test11(struct ptr_no_attr *p) {
   _Static_assert(_Generic(__builtin_counted_by_ref(p->ptr), void * : 1, default : 0) == 1, "should be void*");
 }
+
+struct bitfield_counter {
+  int count : 13; // expected-note {{counter field 'count' declared here}}
+  int array[] __attribute__((counted_by(count)));
+};
+
+void test_bitfield_counter(struct bitfield_counter *p) {
+  __builtin_counted_by_ref(p->array); // expected-error {{'__builtin_counted_by_ref' cannot be used on 'p->array' because its counter field is a bit-field}}
----------------
delcypher wrote:

I'm not a big fan of the "counter field" terminology as it's not something we normally use in `-fbounds-safety`.

How about wording it like this?

```
error: '__builtin_counted_by_ref' cannot be used on 'p->array' because its 'counted_by' attribute refers to bit field 'count'
```

note: 'count' declared here.

I think this is better because

* We aren't introducing new terminology ("counter field").
* We explain that this comes from the `counted_by` attribute which the previous wording never mentioned.

https://github.com/llvm/llvm-project/pull/231591