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

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

This mostly makes sense to me, @AaronBallman does this look good to you?

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