[gdb/cli] Clear line buffer on ^C

Gdb has the ability to gather input over several lines [1], for instance this:
...
(gdb) print 1
$1 = 1
(gdb)
...
can also be typed as:
...
(gdb) print\
 1
$2 = 1
(gdb)
...

Furthermore, if we type a command but change our mind, we can abort using ^C
and start over using a fresh gdb prompt [2]:
...
(gdb) print 1❌️ Quit
(gdb) echo 1\n
1
(gdb)
...

Now say we type a multi-line command but abort it, we get:
...
(gdb) print\
 1❌️ Quit
(gdb) echo 1\n
❌️ Undefined command: "printecho".  Try "help".
(gdb)
...

Using "set trace-commands on", we can see what happened:
...
+printecho 1\n
..

Gdb has prepended the first line of the cancelled multi-line command to the
following command.

Fix this by clearing current_ui->line_buffer on catching a gdb_exception in
start_event_loop.

Tested on x86_64-linux.

Approved-By: Andrew Burgess <aburgess@redhat.com>

PR cli/33063
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33063

[1] https://sourceware.org/gdb/current/onlinedocs/gdb.html/Output.html
[2] https://sourceware.org/gdb/current/onlinedocs/gdb.html/Quitting-GDB.html

gdb/main.c

@@ -420,6 +420,7 @@ start_event_loop ()
 	     get around to resetting the prompt, which leaves readline
 	     in a messed-up state.  Reset it here.  */
 	  current_ui->prompt_state = PROMPT_NEEDED;
+	  current_ui->line_buffer.clear ();
 	  top_level_interpreter ()->on_command_error ();
 	  /* This call looks bizarre, but it is required.  If the user
 	     entered a command that caused an error,

gdb/testsuite/gdb.base/command-line-input.exp

@@ -21,6 +21,9 @@
 gdb_exit
 gdb_start
 
+set bs "\\"
+set re_bs [string_to_regexp $bs]
+
 set test "print 1\\\\n + 2"
 gdb_test_multiple "print 1\\\n + 2" $test {
     -re "^print 1\\\\\r\n \\+ 2\r\n\\\$$decimal = 3\r\n$gdb_prompt $" {
@@ -34,3 +37,30 @@ gdb_test_multiple "print 1\\\n2" $test {
 	pass $test
     }
 }
+
+with_test_prefix "cancel multiline" {
+    send_gdb "print$bs\n 1"
+    gdb_test_multiple "" "setup" {
+	-re "print$re_bs\r\n 1" {
+	    pass $gdb_test_name
+	}
+    }
+
+    send_gdb "\003"
+    gdb_test_multiple "" "cancel" {
+	-re -wrap "" {
+	    pass $gdb_test_name
+	}
+    }
+
+    # Regression test for PR cli/33063.
+    gdb_test_multiple "print 2" "command after cancel" {
+	-re -wrap " = 2" {
+	    pass $gdb_test_name
+	}
+	-re -wrap "" {
+	    # Avoid undefined command error.
+	    fail $gdb_test_name
+	}
+    }
+}