Bitcast versus GEP in LLVM output

From @smeenai:

> `gep` is emitted [here](https://github.com/llvm/clangir/blob/main/clang/lib/CodeGen/CGCall.cpp#L1191) in the original codeden, but we don't do it so far  - we have a missing feature guard

_Originally posted by @gitoleg in https://github.com/llvm/clangir/pull/1041#discussion_r1826530949_