[msl] Update generate_bindings to handle Overrides.

This CL updates the GenerateBindings for the MSL backend to allow more
then just `Var` entries at module scope.

Bug: 374971092
Change-Id: I34af66d39a3508c60de5d657bbc54717b2d43b16
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/214176
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
diff --git a/src/tint/lang/msl/writer/helpers/generate_bindings.cc b/src/tint/lang/msl/writer/helpers/generate_bindings.cc
index 1193e0d..e0ca733 100644
--- a/src/tint/lang/msl/writer/helpers/generate_bindings.cc
+++ b/src/tint/lang/msl/writer/helpers/generate_bindings.cc
@@ -57,6 +57,10 @@
     Vector<tint::BindingPoint, 4> ext_tex_bps;
     for (auto* inst : *module.root_block) {
         auto* var = inst->As<core::ir::Var>();
+        if (!var) {
+            continue;
+        }
+
         if (auto bp = var->BindingPoint()) {
             if (auto val = group_to_next_binding_number.Get(bp->group)) {
                 *val = std::max(*val, bp->binding + 1);