[Macros] Terminate macro plugin

Instead of expecting the plugin read `EOF` and exit, proactively
terminate the plugin and `wait` to reap it.

Author: rintaro

lib/AST/PluginRegistry.cpp

@@ -213,6 +213,7 @@ LoadedExecutablePlugin::PluginProcess::~PluginProcess() {
 #else
   close(input);
   close(output);
+  kill(process.Pid, SIGTERM);
 #endif
 
   // Set `SecondsToWait` non-zero so it waits for the timeout and kill it after