[clang] [clang][ExprConst] allow single element access of vector object to be constant expression (PR #101126)

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

clang crashes compiling the following code.

```
typedef float float4 __attribute__((ext_vector_type(4)));
constexpr float4 v{1,2,3,4};
constexpr const float4 *p = &v;
constexpr float f = p->x;
```

I created a PR that fixes the crash: https://github.com/llvm/llvm-project/pull/136771

https://github.com/llvm/llvm-project/pull/101126