tint/ir: Fix build

Destructors needs to be marked 'override', with no 'virtual'

Change-Id: I8648a160e6564c391b2c8be6ec6c7d02927c1707
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/112382
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Auto-Submit: Ben Clayton <bclayton@google.com>

src/tint/ir/instruction.h

@@ -68,7 +68,7 @@ class Instruction : public Castable<Instruction> {
     /// @param instr the instruction to move from
     Instruction(Instruction&& instr);
     /// Destructor
-    ~Instruction();
+    ~Instruction() override;
 
     /// Copy assign
     /// @param instr the instruction to copy from

src/tint/ir/value.h

@@ -25,7 +25,7 @@ namespace tint::ir {
 class Value : public Castable<Value> {
   public:
     /// Destructor
-    virtual ~Value();
+    ~Value() override;
 
     Value(const Value&) = delete;
     Value(Value&&) = delete;