[llvm] [mlir] [LLVM][TableGen] Change `RecordKeeper::getClass` to return const pointer (PR #112261)

[Jakub Kuderski via llvm-commits]

https://github.com/kuhar commented:

I wonder if we should expand on the 'const correctness' coding style rules before propagating const across these APIs? IR constructs are not designed for [const correctness](https://mlir.llvm.org/docs/Rationale/UsageOfConst/).

In the past @Mogball argued that this was non-goal even for types outside of the core IR.

https://github.com/llvm/llvm-project/pull/112261