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.

DevTools/HackStudio/TerminalWrapper.cpp

@@ -107,13 +107,13 @@ void TerminalWrapper::run_command(const String& command)
         tcsetpgrp(pts_fd, getpid());
 
         // NOTE: It's okay if this fails.
-        (void)ioctl(0, TIOCNOTTY);
+        int rc = ioctl(0, TIOCNOTTY);
 
         close(0);
         close(1);
         close(2);
 
-        int rc = dup2(pts_fd, 0);
+        rc = dup2(pts_fd, 0);
         if (rc < 0) {
             perror("dup2");
             exit(1);
@@ -164,7 +164,7 @@ void TerminalWrapper::kill_running_command()
     ASSERT(m_pid != -1);
 
     // Kill our child process and its whole process group.
-    (void)killpg(m_pid, SIGTERM);
+    [[maybe_unused]] auto rc = killpg(m_pid, SIGTERM);
 }
 
 TerminalWrapper::TerminalWrapper(bool user_spawned)

DevTools/HackStudio/WidgetTool.cpp

@@ -29,27 +29,23 @@
 
 namespace HackStudio {
 
-void WidgetTool::on_mousedown(GUI::MouseEvent& event)
+void WidgetTool::on_mousedown([[maybe_unused]] GUI::MouseEvent& event)
 {
-    (void)event;
     dbgln("WidgetTool::on_mousedown");
 }
 
-void WidgetTool::on_mouseup(GUI::MouseEvent& event)
+void WidgetTool::on_mouseup([[maybe_unused]] GUI::MouseEvent& event)
 {
-    (void)event;
     dbgln("WidgetTool::on_mouseup");
 }
 
-void WidgetTool::on_mousemove(GUI::MouseEvent& event)
+void WidgetTool::on_mousemove([[maybe_unused]] GUI::MouseEvent& event)
 {
-    (void)event;
     dbgln("WidgetTool::on_mousemove");
 }
 
-void WidgetTool::on_keydown(GUI::KeyEvent& event)
+void WidgetTool::on_keydown([[maybe_unused]] GUI::KeyEvent& event)
 {
-    (void)event;
     dbgln("WidgetTool::on_keydown");
 }
 

DevTools/Profiler/Profile.cpp

@@ -95,9 +95,8 @@ static String symbolicate(FlatPtr eip, const ELF::Core::MemoryRegionInfo* region
     return String::format("[%s] %s", name.characters(), lib_data->lib_elf->symbolicate(eip - region->region_start, &offset).characters());
 }
 
-static String symbolicate_from_coredump(CoreDumpReader& coredump, u32 ptr, u32& offset)
+static String symbolicate_from_coredump(CoreDumpReader& coredump, u32 ptr, [[maybe_unused]] u32& offset)
 {
-    (void)offset;
     auto* region = coredump.region_containing((FlatPtr)ptr);
     if (!region) {
         dbgln("did not find region for eip: {:p}", ptr);

DevTools/UserspaceEmulator/Emulator.cpp

@@ -1149,10 +1149,8 @@ int Emulator::virt$get_dir_entries(int fd, FlatPtr buffer, ssize_t size)
     return rc;
 }
 
-int Emulator::virt$ioctl(int fd, unsigned request, FlatPtr arg)
+int Emulator::virt$ioctl([[maybe_unused]] int fd, unsigned request, [[maybe_unused]] FlatPtr arg)
 {
-    (void)fd;
-    (void)arg;
     if (request == TIOCGWINSZ) {
         struct winsize ws;
         int rc = syscall(SC_ioctl, fd, TIOCGWINSZ, &ws);
@@ -1493,8 +1491,8 @@ void Emulator::dispatch_one_pending_signal()
 }
 
 // Make sure the compiler doesn't "optimize away" this function:
-extern void signal_trampoline_dummy(void);
-void signal_trampoline_dummy(void)
+extern void signal_trampoline_dummy();
+void signal_trampoline_dummy()
 {
     // The trampoline preserves the current eax, pushes the signal code and
     // then calls the signal handler. We do this because, when interrupting a
@@ -1518,8 +1516,8 @@ void signal_trampoline_dummy(void)
         ".att_syntax" ::"i"(Syscall::SC_sigreturn));
 }
 
-extern "C" void asm_signal_trampoline(void);
-extern "C" void asm_signal_trampoline_end(void);
+extern "C" void asm_signal_trampoline();
+extern "C" void asm_signal_trampoline_end();
 
 void Emulator::setup_signal_trampoline()
 {