[clang] Add CodeGen tests for CWG 6xx issues (#87876)

This patch covers
[CWG605](https://cplusplus.github.io/CWG/issues/605.html) "Linkage of
explicit specializations",
[CWG650](https://cplusplus.github.io/CWG/issues/650.html) "Order of
destruction for temporaries bound to the returned value of a function",
[CWG653](https://cplusplus.github.io/CWG/issues/653.html) "Copy
assignment of unions",
[CWG658](https://cplusplus.github.io/CWG/issues/658.html) "Defining
`reinterpret_cast` for pointer types",
[CWG661](https://cplusplus.github.io/CWG/issues/661.html) "Semantics of
arithmetic comparisons",
[CWG672](https://cplusplus.github.io/CWG/issues/672.html) "Sequencing of
initialization in _new-expression_s".

[CWG624](https://cplusplus.github.io/CWG/issues/624.html) "Overflow in
calculating size of allocation" and
[CWG668](https://cplusplus.github.io/CWG/issues/668.html) "Throwing an
exception from the destructor of a local static object" are marked as
requiring libc++abi tests.