[clang] [clang] Support constexpr bitcasts between equal-size vector and integer types (PR #167016)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

You should be able to create a generic test without including anything:
```cpp
typedef long long __m64 __attribute__((__vector_size__(8), __aligned__(8)));

constexpr __m64 foo(__m64 a, int b) {
    return (__m64)(long long)a << b;
}

```

https://github.com/llvm/llvm-project/pull/167016