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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> ```
> typedef __attribute__((__ext_vector_type__(2))) float float2;
> 
> struct vec2 {
>     float2 _v;
> public:
>     constexpr vec2(float x, float y) {
>        _v.x = x;
>        _v.y = y;
>     }
> };
> 
> constexpr struct vec2 f() {
>   return vec2(1.0, 1.0);
> }
> 
> int main() {
>     vec2 S = f();
> }
> ```
> 
> Clang crashes: https://godbolt.org/z/sx74s1bqd

Can confirm the crash is happening. The issue is related to the constructor being `constexpr` and not initializing the member variable. If you use a member initializer list, the assert is silenced: https://godbolt.org/z/vEbeb4jbz

Can you file an issue so we don't lose track of the bug? Also @vikramRH any chance you can look into this?

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