add several noreturn statements

Found with clang's -Wmissing-noreturn

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/env_universal_common.cpp

@@ -1178,7 +1178,7 @@ class universal_notifier_shmem_poller_t : public universal_notifier_t {
     }
 #else  // this class isn't valid on this system
    public:
-    universal_notifier_shmem_poller_t() {
+    [[noreturn]] universal_notifier_shmem_poller_t() {
         DIE("universal_notifier_shmem_poller_t cannot be used on this system");
     }
 #endif
@@ -1257,7 +1257,7 @@ class universal_notifier_notifyd_t : public universal_notifier_t {
     }
 #else  // this class isn't valid on this system
    public:
-    universal_notifier_notifyd_t() {
+    [[noreturn]] universal_notifier_notifyd_t() {
         DIE("universal_notifier_notifyd_t cannot be used on this system");
     }
 #endif

src/exec.cpp

@@ -87,6 +87,7 @@ pgroup_provenance_t get_pgroup_provenance(const shared_ptr<job_t> &j,
 /// This function is executed by the child process created by a call to fork(). It should be called
 /// after \c child_setup_process. It calls execve to replace the fish process image with the command
 /// specified in \c p. It never returns. Called in a forked child! Do not allocate memory, etc.
+[[noreturn]]
 static void safe_launch_process(process_t *p, const char *actual_cmd, const char *const *cargv,
                                 const char *const *cenvv) {
     UNUSED(p);
@@ -132,6 +133,7 @@ static void safe_launch_process(process_t *p, const char *actual_cmd, const char
 
 /// This function is similar to launch_process, except it is not called after a fork (i.e. it only
 /// calls exec) and therefore it can allocate memory.
+[[noreturn]]
 static void launch_process_nofork(env_stack_t &vars, process_t *p) {
     ASSERT_IS_MAIN_THREAD();
     ASSERT_IS_NOT_FORKED_CHILD();

src/fish_key_reader.cpp

@@ -242,6 +242,7 @@ static void process_input(bool continuous_mode) {
 }
 
 /// Setup our environment (e.g., tty modes), process key strokes, then reset the environment.
+[[noreturn]]
 static void setup_and_process_keys(bool continuous_mode) {
     set_interactive_session(session_interactivity_t::implied);
     set_main_thread();