[clang] [BoundsSafety][Sema] Allow counted_by and counted_by_or_null on pointers where the pointee type is incomplete but potentially completable (PR #106321)

[Dan Liew via cfe-commits]

================
@@ -19,13 +19,12 @@ struct on_member_pointer_complete_ty {
 };
 
 struct on_member_pointer_incomplete_ty {
-  struct size_unknown * buf __counted_by(count); // expected-error{{'counted_by' cannot be applied to a pointer with pointee of unknown size because 'struct size_unknown' is an incomplete type}}
+  struct size_unknown * buf __counted_by(count); // ok
----------------
delcypher wrote:

@AaronBallman I should have also noted the current design also allows for headers that deliberately don't provide complete types to reduce compile times which I believe is fairly common practice.

E.g.

```
// some_data.h

struct SomeData {
 int value;
};

// list_handle.h

// Forward declare to avoid needing to include `some_data.h`
struct SomeData;

struct ListHandle {
  struct SomeData* __counted_by(count) items;
  size_t count;
};
```


https://github.com/llvm/llvm-project/pull/106321