Translate `-emit-opaque-pointers` to LLVM's -opaque-pointers (LLVM 16)

Please correct me if I'm wrong, but the `emit-opaque-pointers` flag seems redundant. It overlaps with the existing LLVM `-opaque-pointers` flag:

* `-opaque-pointers` is provided by LLVM, enabled by default, and controls whether opaque or typed pointers are used in both directions (LLVM to SPIR-V and SPIR-V to LLVM).
* `emit-opaque-pointers` is a hidden flag we define, and it only affects the SPIR-V to LLVM direction.

We don't maintain `-opaque-pointers` ourselves — it’s part of LLVM. Having a second flag with overlapping semantics introduces complexity and inconsistency.

### On Inconsistent Behavior

Right now, opaque pointers are used when translating LLVM to SPIR-V, but typed pointers are used when going from SPIR-V to LLVM. This inconsistency can lead to confusing behavior, especially since LLVM doesn't verify that the IR matches the pointer type mode. If there's a mismatch, it can result in undefined behavior or subtle bugs.

Also, we can't safely enable `emit-opaque-pointers` by default. An [assertion in LLVMContext](https://github.com/llvm/llvm-project/blob/7cbf1a2591520c2491aa35339f227775f4d3adf6/llvm/lib/IR/LLVMContextImpl.cpp#L259) prevents disabling opaque pointers once they’ve been enabled. So if the flag defaults to true and someone tries to disable it, it will crash.

### Proposal

Rather than removing `emit-opaque-pointers` outright (which would be a breaking change even though it's hidden), we could:

* Translate `emit-opaque-pointers` into LLVM's `-opaque-pointers` internally.
* Always enable opaque pointers by default to match LLVM's default and future behavior.
* Ensure we use opaque pointers by default in both translation directions, including SPIR-V to LLVM.
* Disallow setting both flags to different values at the same time.

This would avoid configuration inconsistencies while preserving backward compatibility.

Open to other suggestions, but this seems like a clean, non-breaking step in the right direction.

Start by locating the existing hidden `emit-opaque-pointers` flag and the handling of LLVM's `-opaque-pointers`; the issue does not name repository files or tests. Read LLVMContextImpl.cpp at the referenced assertion, then trace both LLVM-to-SPIR-V and SPIR-V-to-LLVM entry points. Done means consistent defaults, compatibility translation, and rejection of conflicting flag values without removing the existing hidden flag.

Written by the indexing model from the issue text.