Clang rejects gcc extension of allowing vector reinterpret_cast in constant expressions

[Test case](https://compiler-explorer.com/z/Eh4erPMsq):

```c++
using V0 [[gnu::vector_size(16)]] = int;
using V1 [[gnu::vector_size(16)]] = float;

constexpr V0 x = reinterpret_cast(V1());
```

GCC set the precedent of bit-casting vectors via `reinterpret_cast`. It does so with additional semantic information (i.e. higher quality diagnostics on errors), which is why the libstdc++ std::simd implementation prefers `reinterpret_cast` over `std::bit_cast`. However, Clang rejects these casts in constant expressions, which is incompatible to GCC.

Proposed resolution: Allow "reinterpret-cast by value of GNU vectors" in constant expressions.

Start with the linked Compiler Explorer test case and inspect Clang's constant-expression handling for GNU vector reinterpret_cast expressions. Compare the behavior with GCC and the stated libstdc++ std::simd use case. Done means the shown constexpr cast is accepted by Clang while invalid cases retain useful diagnostics.

Written by the indexing model from the issue text.