[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

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