Clang rejects conditional operator on GNU vectors in constant expressions

[Test case](https://compiler-explorer.com/z/68GEcjrE8):

```c++
using V [[gnu::vector_size(16)]] = int;
constexpr V x = V() == V() ? V() : V();
```

There is no reason to not support the conditional operator in constant expressions of GNU vector types if all the other operators are supported. And, obviously, this issue blocks the use of libstdc++'s std::simd implementation.

Start with the Compiler Explorer test case linked in the issue and reproduce the constexpr failure for the GNU vector conditional expression. Trace Clang's handling of constant expressions and GNU vector operators, then add coverage showing that the expression is accepted while preserving the expected behavior for unsupported cases.

Written by the indexing model from the issue text.