[llvm] [mlir] [CMake][TableGen] Make TableGen CMake functions compatible with CMP0116 (PR #72333)

[NAKAMURA Takumi via llvm-commits]

https://github.com/chapuni requested changes to this pull request.

I suggest, in this opportunity;

* Drop `cmake_policy(SET CMP0116 OLD)` to follow the new policy
* Drop the old logic
* Anyways check if `CMP0116` is `NEW`, otherwise use the fallback logic.

I suppose users of LLVM (as a module) may assume using CMake>=3.20.

https://github.com/llvm/llvm-project/pull/72333