[Clang] bogus error about vector values of different size

With `-flax-vector-conversions=none`, clang rejects the following code:
```c++
template
using vec [[clang::ext_vector_type(4)]] = T;

auto test(vec v, vec v2) {
return v == v2;
}
```

I'm not quite sure whether it's intended to work, but the diagnostic message is definitely bogus:
```
:6:12: error: cannot convert between vector values of different size ('vec' (vector of 4 'int' values) and 'vec' (vector of 4 'const int' values))
6 | return v == v2;
| ~ ^ ~~
```
Both the element size and total vector size are identical.

Reproduce the reduced C++ example with -flax-vector-conversions=none and compare the reported vector sizes for vec and vec. Trace the diagnostic path that handles the v == v2 comparison; done means the diagnostic no longer claims the vectors have different sizes, with behavior matching the intended handling of these vector types.

Written by the indexing model from the issue text.