tint: Add forward declaration for CastableBase

This fixes the google3 build, which compiles headers in isolation.

Change-Id: I3c9a91caa75c6da1d9dcc17bb6d2aeb5834c197e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/115440
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Auto-Submit: James Price <jrprice@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

src/tint/sem/type_mappings.h

@@ -18,6 +18,9 @@
 #include <type_traits>
 
 // Forward declarations
+namespace tint {
+class CastableBase;
+}  // namespace tint
 namespace tint::ast {
 class Array;
 class Expression;