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

[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
----------------
Sirraide wrote:

Yeah, that makes sense. Being able to benefit from the attribute in the implementation of a library without requiring a type to be complete at the API level seems like a reasonable use case.

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