[clang][diverges-from:gcc][ABI][x86-64] ABI mismatch from GCC when using vector type members with an empty base class

Godbolt link: https://godbolt.org/z/84EProx4c. Compile options: `-Og -march=x86-64-v3`.

```c++
#include
template
using vec [[gnu::vector_size(N * sizeof(T))]] = T;
using data_t = vec;

struct Base { };
struct A : Base {
[[no_unique_address]]
data_t data;
};

A f() {
return A{};
}

std::uint64_t f2(A x) {
return x.data[0];
}
```

When using an empty base class, clang passes and returns `A` on stack, but gcc passes and returns `A` in register. Removing the base class makes clang passes and returns `A` in register.

Prossibly related: #183723.

Start with the Godbolt reproducer using clang and GCC with -Og -march=x86-64-v3, then investigate Clang's x86-64 ABI classification for vector-type members, empty bases, and [[no_unique_address]]. Done means Clang passes and returns A in registers, matching GCC, while preserving the behavior when the base class is removed.

Written by the indexing model from the issue text.