[clang] [CIR] Upstream pointer subtraction handling (PR #163306)

[Andy Kaylor via cfe-commits]

================
@@ -1734,9 +1734,9 @@ mlir::Value ScalarExprEmitter::emitSub(const BinOpInfo &ops) {
   // LLVM we shall take VLA's, division by element size, etc.
   //
   // See more in `EmitSub` in CGExprScalar.cpp.
-  assert(!cir::MissingFeatures::ptrDiffOp());
-  cgf.cgm.errorNYI("ptrdiff");
-  return {};
+  assert(!cir::MissingFeatures::llvmLoweringPtrDiffConsidersPointee());
+  return builder.create<cir::PtrDiffOp>(cgf.getLoc(ops.loc), cgf.PtrDiffTy,
+                                        ops.lhs, ops.rhs);
----------------
andykaylor wrote:

```suggestion
  return cir::PtrDiffOp::create(builder, cgf.getLoc(ops.loc), cgf.PtrDiffTy,
                                        ops.lhs, ops.rhs);
```
This is a change to the preferred operation creation idiom that hasn't yet been implemented in all the incubator code.

https://github.com/llvm/llvm-project/pull/163306