tint/ast: Add missing 'const' on pointer member

Change-Id: I8572e3b3f1b26ba79b1306e23f3447cc739108a4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/104660
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>

src/tint/ast/if_statement.h

@@ -55,7 +55,7 @@ class IfStatement final : public Castable<IfStatement, Statement> {
     const BlockStatement* const body;
 
     /// The optional else statement, or nullptr
-    const Statement* else_statement;
+    const Statement* const else_statement;
 };
 
 }  // namespace tint::ast