Add attribute [[noreturn]] (C++11) to functions that will not return

Rationale:
* Reduce the number of false positives from static analyzers
* Potentially enable additional compiler optimizations

src/test/test_bitcoin_main.cpp

@@ -10,14 +10,14 @@
 
 std::unique_ptr<CConnman> g_connman;
 
-void Shutdown(void* parg)
+[[noreturn]] void Shutdown(void* parg)
 {
-  exit(EXIT_SUCCESS);
+  std::exit(EXIT_SUCCESS);
 }
 
-void StartShutdown()
+[[noreturn]] void StartShutdown()
 {
-  exit(EXIT_SUCCESS);
+  std::exit(EXIT_SUCCESS);
 }
 
 bool ShutdownRequested()