Everywhere: Switch from (void) to [[maybe_unused]] (#4473)

Problem:
- `(void)` simply casts the expression to void. This is understood to
  indicate that it is ignored, but this is really a compiler trick to
  get the compiler to not generate a warning.

Solution:
- Use the `[[maybe_unused]]` attribute to indicate the value is unused.

Note:
- Functions taking a `(void)` argument list have also been changed to
  `()` because this is not needed and shows up in the same grep
  command.

Libraries/LibJS/Forward.h

@@ -42,7 +42,7 @@
     JS::Value name([[maybe_unused]] JS::VM& vm, [[maybe_unused]] JS::GlobalObject& global_object)
 
 #define JS_DEFINE_NATIVE_SETTER(name) \
-    void name([[maybe_unused]] JS::VM& vm, [[maybe_unused]] JS::GlobalObject& global_object, JS::Value value)
+    void name([[maybe_unused]] JS::VM& vm, [[maybe_unused]] JS::GlobalObject& global_object, [[maybe_unused]] JS::Value value)
 
 // NOTE: Proxy is not included here as it doesn't have a prototype - m_proxy_constructor is initialized separately.
 #define JS_ENUMERATE_NATIVE_OBJECTS_EXCLUDING_TEMPLATES                                           \

Libraries/LibJS/Runtime/Accessor.h

@@ -63,7 +63,7 @@ public:
         if (!m_setter)
             return;
         // FIXME: It might be nice if we had a way to communicate to our caller if an exception happened after this.
-        (void)vm().call(*m_setter, this_value, setter_value);
+        [[maybe_unused]] auto rc = vm().call(*m_setter, this_value, setter_value);
     }
 
     void visit_edges(Cell::Visitor& visitor) override

Libraries/LibJS/Runtime/IteratorOperations.cpp

@@ -87,9 +87,8 @@ Object* iterator_next(Object& iterator, Value value)
     return &result.as_object();
 }
 
-void iterator_close(Object& iterator)
+void iterator_close([[maybe_unused]] Object& iterator)
 {
-    (void)iterator;
     TODO();
 }
 

Libraries/LibJS/Runtime/VM.h

@@ -225,10 +225,7 @@ public:
     template<typename... Args>
     [[nodiscard]] ALWAYS_INLINE Value call(Function& function, Value this_value, Args... args)
     {
-        // Are there any values in this argpack?
-        // args = [] -> if constexpr (false)
-        // args = [x, y, z] -> if constexpr ((void)x, true || ...)
-        if constexpr ((((void)args, true) || ...)) {
+        if constexpr (sizeof...(Args) > 0) {
             MarkedValueList arglist { heap() };
             (..., arglist.append(move(args)));
             return call(function, this_value, move(arglist));