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

[Ziqing Luo via cfe-commits]

ziqingluo-90 wrote:

> Can you file an issue so we don't lose track of the bug?
Looks like we have an issue already #140348

@AaronBallman do you think my original example is well-defined according to the standard?  I saw there is a warning
`constexpr constructor that does not initialize all members is a C++20 extension [-Wc++20-extensions]`.  




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