[llvm] [llvm] Remove "const" in the presence of "constexpr" (NFC) (PR #166109)

[Shilei Tian via llvm-commits]

https://github.com/shiltian approved this pull request.

If my memory serves me correctly, at one point, a pretty famous C++ book recommended writing `constexpr const`, but I can't remember which one. I also write them as `constexpr const`.

https://github.com/llvm/llvm-project/pull/166109