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

[Craig Topper via cfe-commits]

topperc wrote:

Do Init objects have any methods that can modify them? They const was explicitly removed from some uses a long time ago af8ee2cdeeb2ed35fdf0db8f7c06254c5d333ac4.

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