[clang] [clang][ExprConst] allow single element access of vector object to be constant expression (PR #72607)

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https://github.com/sethp commented:

Thanks for looking into this! I was surprised to find that `clang` didn't support element accesses in a `constexpr` context, I'm glad you've put the effort in to get it working!

Two minor notes below, otherwise LGTM.

https://github.com/llvm/llvm-project/pull/72607