[llvm] [mlir] Fix LLVM_ENABLE_ABI_BREAKING_CHECKS macro check: use #if instead of #ifdef (PR #110185)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

I don't think I need more repro steps, but you likely wanna check what your custom cmake ends up with LLVM inferring LLVM_ABI_BREAKING_CHECKS to be on?

https://github.com/llvm/llvm-project/pull/110185