[tint][ir] Serialize Switch instructions

Change-Id: I3741dd6cdde76ad8f2773d6c1011aa8cc372fcce
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/165004
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
diff --git a/src/tint/lang/core/ir/exit_switch.h b/src/tint/lang/core/ir/exit_switch.h
index 878f73b..d7d3032 100644
--- a/src/tint/lang/core/ir/exit_switch.h
+++ b/src/tint/lang/core/ir/exit_switch.h
@@ -46,6 +46,9 @@
     /// The base offset in Operands() for the args
     static constexpr size_t kArgsOperandOffset = 0;
 
+    /// Constructor (no operands, no switch)
+    ExitSwitch();
+
     /// Constructor
     /// @param sw the switch being exited
     /// @param args the target MultiInBlock arguments