[Clang] ABI mismatch between Clang and GCC when using vector types in a union

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

union S {
vec v;
vec w[4];
};

S test() {
return S{};
}
```
Clang and GCC disagree on whether the above code passes `S` in a register or on the stack.

Start by compiling and inspecting the supplied C++ reproducer with both Clang and GCC, confirming how the union return value is passed. Then trace Clang's ABI handling for vector types in unions and add a regression test for the case. Done means Clang and GCC agree on whether S is passed in a register or on the stack.

Written by the indexing model from the issue text.