llvm-wrapper: adapt for LLVM API changes

No functional changes intended.

Adapts the wrapper for https://github.com/llvm/llvm-project/commit/0a1aa6cda2758b0926a95f87d39ffefb1cb90200.

Found by our experimental rust + llvm @ head CI: https://buildkite.com/llvm-project/rust-llvm-integrate-prototype/builds/22301#018a9926-a810-4e56-8ac2-e6f30b78f433/253-551