[clang] [mlir][llvm] Fix elem type passing into `getelementptr` (PR #68136)

[Stella Laurenzo via cfe-commits]

stellaraccident wrote:

Thank you for the revert.

We were failing on assertion when integrating this patch:

```
iree-compile: /home/stella/megabump/work/iree/third_party/llvm-project/llvm/include/llvm/Support/Casting.h:566: decltype(auto) llvm::cast(const From &) [To = mlir::LLVM::LLVMPointerType, F
rom = mlir::Type]: Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
```

https://github.com/llvm/llvm-project/pull/68136