[clang] [llvm] [mlir] [TableGen] Add const variants of accessors for backend (PR #106658)

[Rahul Joshi via cfe-commits]

jurahul wrote:

I posted the PSA at: https://discourse.llvm.org/t/psa-planned-changes-to-tablegen-getallderiveddefinitions-api-potential-downstream-breakages/81089

https://github.com/llvm/llvm-project/pull/106658