tint/msl: Avoid cloning a built-in struct

This prevents us from crashing when a module-scope private variable
has an inferred type that is a built-in struct. We instead fail to
resolve the generated code.

Bug: chromium:1430309
Change-Id: I71c6eacf28096380227ed1043821ecc3d33ffb40
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/129140
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>

src/tint/transform/module_scope_var_to_entry_point_param.cc

@@ -85,6 +85,11 @@ struct ModuleScopeVarToEntryPointParam::State {
                 return;
             }
 
+            if (!str->Declaration()) {
+                // The struct is a built-in structure that we do not need to declare.
+                return;
+            }
+
             // Recurse into members.
             for (auto* member : str->Members()) {
                 CloneStructTypes(member->Type());