[Clang][Sema] Allow counted_by on void* as GNU extension (#164737)

The counted_by attribute currently rejects void* members because void
has no defined size. However, the sized_by attribute accepts void* since
it explicitly measures bytes. As a GNU extension, void pointer
arithmetic treats void as having size 1 byte, so counted_by on void*
should behave identically to sized_by (treating the count as bytes).

Allow counted_by on void* as a GNU extension. The implementation
validates this only at declaration time in SemaBoundsSafety.cpp,
emitting a -Wpointer-arith warning that the attribute is treated as a
GNU extension equivalent to sized_by. Both use-site validation and code
generation trust this earlier validation, avoiding redundant checks.

In CodeGen, __builtin_dynamic_object_size now correctly handles
counted_by on void* by treating any CountAttributedType with zero
element size as having 1-byte elements, matching the GNU void pointer
arithmetic semantics.

Add tests validating both Sema diagnostics and CodeGen behavior (correct
byte counts from __builtin_dynamic_object_size). Update existing
counted_by tests to explicitly use -Wpointer-arith to preserve their
original intent of rejecting void* in strict C mode.