[Macros] Send termination signal to macro plugin

When shutting down, the plugin, we send SIGTERM, then wait(2) on it. But
we observed some cases the compiler waits for the plugin process exit
for long time.

To resolve that, send an empty message to the plugin and let them exit
itself.

rdar://160820381

Author: rintaro

include/swift/AST/PluginRegistry.h

@@ -179,6 +179,7 @@ class LoadedExecutablePlugin : public CompilerPlugin {
       : CompilerPlugin(ExecutablePath),
         LastModificationTime(LastModificationTime),
         disableSandbox(disableSandbox){};
+  ~LoadedExecutablePlugin();
 
   /// The last modification time of 'ExecutablePath' when this object is
   /// created.

lib/AST/PluginRegistry.cpp

@@ -172,6 +172,16 @@ PluginRegistry::loadExecutablePlugin(StringRef path, bool disableSandbox) {
   return storage.get();
 }
 
+LoadedExecutablePlugin::~LoadedExecutablePlugin() {
+  if (Process) {
+    // If the process is alive, send an empty message as the termination signal.
+    // The plugin should exit itself when receiving it.
+    if (auto error = sendMessage(""))
+      llvm::consumeError(std::move(error));
+    Process.reset();
+  }
+}
+
 llvm::Error LoadedExecutablePlugin::spawnIfNeeded() {
   if (Process) {
     // NOTE: We don't check the mtime here because 'stat(2)' call is too heavy.