[CIR][CodeGen] Implement union cast (#867)

Currently the C style cast is not implemented/supported for unions. 

This PR adds support for union casts as done in `CGExprAgg.cpp`. I have
also added an extra test in `union-init.c`.

clang/lib/CIR/CodeGen/CIRGenExprAgg.cpp

@@ -980,6 +980,22 @@ void AggExprEmitter::VisitCastExpr(CastExpr *E) {
     break;
   }
 
+  case CK_ToUnion: {
+    // Evaluate even if the destination is ignored.
+    if (Dest.isIgnored()) {
+      CGF.buildAnyExpr(E->getSubExpr(), AggValueSlot::ignored(),
+                       /*ignoreResult=*/true);
+      break;
+    }
+
+    // GCC union extension
+    QualType Ty = E->getSubExpr()->getType();
+    Address CastPtr = Dest.getAddress().withElementType(CGF.ConvertType(Ty));
+    buildInitializationToLValue(E->getSubExpr(),
+                                CGF.makeAddrLValue(CastPtr, Ty));
+    break;
+  }
+
   case CK_LValueToRValue:
     // If we're loading from a volatile type, force the destination
     // into existence.

clang/test/CIR/CodeGen/union-init.c

@@ -45,4 +45,19 @@ unsigned is_little(void) {
 // CHECK: cir.func @is_little
 // CHECK: %[[VAL_1:.*]] = cir.get_global @is_little.one : !cir.ptr<![[anon0]]>
 // CHECK: %[[VAL_2:.*]] = cir.cast(bitcast, %[[VAL_1]] : !cir.ptr<![[anon0]]>), !cir.ptr<![[anon1]]>
-// CHECK: %[[VAL_3:.*]] = cir.get_member %[[VAL_2]][1] {name = "c"} : !cir.ptr<![[anon1]]> -> !cir.ptr<!cir.array<!u8i x 4>>
+// CHECK: %[[VAL_3:.*]] = cir.get_member %[[VAL_2]][1] {name = "c"} : !cir.ptr<![[anon1]]> -> !cir.ptr<!cir.array<!u8i x 4>>
+
+typedef union {
+  int x;
+} U;
+
+// CHECK: %[[VAL_0:.*]] = cir.alloca !s32i, !cir.ptr<!s32i>, ["x", init] {alignment = 4 : i64}
+// CHECK: %[[VAL_1:.*]] = cir.alloca !ty_U, !cir.ptr<!ty_U>, ["u", init] {alignment = 4 : i64}
+// CHECK: cir.store %arg0, %[[VAL_0]] : !s32i, !cir.ptr<!s32i>
+// CHECK: %[[VAL_2:.*]] = cir.load %[[VAL_0]] : !cir.ptr<!s32i>, !s32i
+// CHECK: %[[VAL_3:.*]] = cir.cast(bitcast, %[[VAL_1]] : !cir.ptr<!ty_U>), !cir.ptr<!s32i>
+// CHECK: cir.store %[[VAL_2]], %[[VAL_3]] : !s32i, !cir.ptr<!s32i>
+
+void union_cast(int x) {
+  U u = (U) x;
+}