[TODO] Support opaque pointers in LLVM IR generator

[MaxXSoft]

Currently, LlvmGenerator will generate LLVM IRs with typed pointers, which will be accepted by llc-21 or other tools in the LLVM 21 toolchain, due to the existence of the opaque pointers mode.

However, this is only a transitional solution. We still need to completely abandon typed pointers and only support opaque pointers.

We will support this in the next revision/minor version of koopa.