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

[Rahul Joshi via cfe-commits]

jurahul wrote:

I also did a minor update to the diff: I changed the const versions that used to return references to vectors to instead return `ArrayRef<const Record *>`. IMO this will prevent accidental vector copies in code like `auto X = RC.getAllDerivedDefinitions("yyy")`.

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