[CIR][OpenCL] Support lowering of OCL opaque types

Implement handling for zero-initialization casts to OpenCL opaque types
in CIR. This covers cases like `event_t e = async_work_group_copy(..., 0)`.

- `VisitCastExpr`: CK_ZeroToOCLOpaqueType now returns a null pointer of the
  appropriate opaque type instead of `llvm_unreachable`.
- `CIRGenTypes::convertType`: Added proper CIR type conversions for OpenCL
  opaque types including event, queue, and reserve_id types.
- Provides consistent CIR representation for OpenCL opaque objects.

Author: Mahmood Yassin

clang/lib/CIR/CodeGen/CIRGenExprScalar.cpp

@@ -1945,8 +1945,18 @@ mlir::Value ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
     return emitComplexToScalarConversion(CGF.getLoc(CE->getExprLoc()), V, Kind,
                                          DestTy);
   }
-  case CK_ZeroToOCLOpaqueType:
-    llvm_unreachable("NYI");
+  case CK_ZeroToOCLOpaqueType: {
+    // OpenCL: event_t e = async_work_group_copy(..., 0);
+    // The source is an integer constant zero; the destination is an OpenCL
+    // opaque type
+    mlir::Type destTy = CGF.convertType(DestTy);
+    auto PtrTy =
+        cir::PointerType::get(destTy, cir::AddressSpace::OffloadPrivate);
+    auto constNullPtrAttr = Builder.getConstNullPtrAttr(PtrTy);
+    auto nullVal =
+        Builder.getConstant(CGF.getLoc(E->getExprLoc()), constNullPtrAttr);
+    return nullVal;
+  }
   case CK_IntToOCLSampler:
     llvm_unreachable("NYI");
 

clang/lib/CIR/CodeGen/CIRGenTypes.cpp

@@ -516,8 +516,13 @@ mlir::Type CIRGenTypes::convertType(QualType T) {
     case BuiltinType::OCLEvent:
     case BuiltinType::OCLClkEvent:
     case BuiltinType::OCLQueue:
+      ResultType = Builder.getVoidPtrTy();
+      break;
     case BuiltinType::OCLReserveID:
-      assert(0 && "not implemented");
+      ResultType = cir::RecordType::get(
+          &getMLIRContext(), {},
+          mlir::StringAttr::get(&getMLIRContext(), "ocl_reserve_id"), false,
+          false, cir::RecordType::Struct);
       break;
     case BuiltinType::SveInt8:
     case BuiltinType::SveUint8: