Parse num_workgroups builtin and reject it

We have chosen not to implement this for origin trial, so just parse
it and produce an error in the validator.

Bug: tint:752
Change-Id: I40ebf0b4051fb0b8bab9d4c733e555b836320cd4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/58780
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/resolver/resolver.cc

@@ -1333,6 +1333,10 @@ bool Resolver::ValidateBuiltinDecoration(const ast::BuiltinDecoration* deco,
         return false;
       }
       break;
+    case ast::Builtin::kNumWorkgroups:
+      // TODO(crbug.com/tint/752): Backend support (needs extra work for HLSL).
+      AddError("num_workgroups builtin is not yet implemented", deco->source());
+      return false;
     default:
       break;
   }