Implement DAP 'terminated' event

This implements the DAP 'terminated' event.  Vladimir Makaev noticed
that VSCode will not report the debug session as over unless this is
sent.

It's not completely clear when exactly this event ought to be sent.
Here I've done it when the inferior exits.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30681

gdb/python/lib/gdb/dap/events.py

@@ -32,6 +32,7 @@ def _on_exit(event):
             "exitCode": code,
         },
     )
+    send_event("terminated")
 
 
 @in_gdb_thread

gdb/python/lib/gdb/dap/server.py

@@ -151,7 +151,7 @@ class Server:
         self.done = True
 
 
-def send_event(event, body):
+def send_event(event, body=None):
     """Send an event to the DAP client.
     EVENT is the name of the event, a string.
     BODY is the body of the event, an arbitrary object."""

gdb/testsuite/gdb.dap/terminate.exp

@@ -0,0 +1,39 @@
+# Copyright 2023 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test that the 'terminated' event is delivered.
+
+require allow_dap_tests
+
+load_lib dap-support.exp
+
+# The test doesn't matter much, since we aren't going to set any
+# breakpoints -- it just has to exit.
+standard_testfile scopes.c
+
+if {[build_executable ${testfile}.exp $testfile $srcfile] == -1} {
+    return
+}
+
+if {[dap_launch $testfile] == ""} {
+    return
+}
+
+dap_check_request_and_response "start inferior" configurationDone
+dap_wait_for_event_and_check "inferior started" thread "body reason" started
+
+dap_wait_for_event_and_check "terminated event" terminated
+
+dap_shutdown