[llvm] Fix memory leak in LLVMTargetMachine.cpp (PR #109610)

[via llvm-commits]

abhishek-kaushik22 wrote:

@MaskRay Can you please review? In a previous commit, you made `MAB` a raw pointer from a `unique_ptr`. If this is not needed, please close.

https://github.com/llvm/llvm-project/pull/109610