Replace trivial copy constructor call with cir::CopyOp

In ClangIR, we replaced a trivial copy constructor call with cir::CopyOp in the LoweringPrepare pass. `cir::CopyOp` is already upstreamed, but not used for that case.

Code in ClangIR: https://github.com/llvm/clangir/blob/823e943d1b9aaba0fc46f880c5a6ac8c29fc761d/clang/lib/CIR/Dialect/Transforms/LoweringPrepare.cpp#L1537-L1550

Ref: https://github.com/llvm/llvm-project/pull/160999#discussion_r2388854615