Fix sem::Node forward declare

To unblock g3 tint import

Change-Id: Id6591dc210e06c93ca09f15056ba5cbccae7d2a9
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/70703
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Shrek Shao <shrekshao@google.com>

src/sem/type_mappings.h

@@ -41,6 +41,7 @@ class Call;
 class Expression;
 class Function;
 class MemberAccessorExpression;
+class Node;
 class Statement;
 class Struct;
 class StructMember;