[llvm] [mlir] [CMake] Do not set CMP0116 explicitly to old (PR #90385)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> @mstorsjo This will introduce the suboptimal "fallback" behavior to Windows developers who use older version of CMake. Do you think it would be acceptable?

I haven't followed it very closely what this practically is all about, but from a quick 1 minute read of the CMP0116 policy, it seems totally reasonable to keep it set to the old behaviour conditionally for windows, with old cmake.

https://github.com/llvm/llvm-project/pull/90385