[clang] Add CodeGen tests for CWG 5xx issues (#84303)
This patch covers
[CWG519](https://cplusplus.github.io/CWG/issues/519.html) "Null pointer
preservation in `void*` conversions",
[CWG571](https://cplusplus.github.io/CWG/issues/571.html) "References
declared const".
