util: throw if unreachable code is reached

If a comparison code path that is supposed to be unreachable is reached,
throw. Add a c8 comment to ignore coverage for the line, as it should be
unreachable.

PR-URL: #31712
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Yosuke Furukawa <yosuke.furukawa@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/internal/util/comparisons.js

@@ -23,6 +23,7 @@ const {
 } = primordials;
 
 const { compare } = internalBinding('buffer');
+const assert = require('internal/assert');
 const types = require('internal/util/types');
 const {
   isAnyArrayBuffer,
@@ -118,7 +119,8 @@ function isEqualBoxedPrimitive(val1, val2) {
     return isSymbolObject(val2) &&
           SymbolPrototypeValueOf(val1) === SymbolPrototypeValueOf(val2);
   }
-  return false;
+  /* c8 ignore next */
+  assert.fail(`Unknown boxed type ${val1}`);
 }
 
 function isIdenticalTypedArrayType(a, b) {