[clang] [llvm] [mlir] [LLVM][TableGen] Change all `Init` pointers to const (PR #112705)

[Rahul Joshi via cfe-commits]

jurahul wrote:

Also, this enabled removing one const_cast<> in https://github.com/llvm/llvm-project/pull/112261 (which I think is going to be the final change for the time being to make more pointers const in TableGen backends).

https://github.com/llvm/llvm-project/pull/112705