[gdb] Only force INTERP_CONSOLE ui_out for breakpoint commands in MI mode

[gdb] Only force INTERP_CONSOLE ui_out for breakpoint commands in MI mode

The problem reported in PR mi/25055 is that the output of the backtrace
command, when executed as breakpoint command does not show when executing
using the MI interpreter:
...
$ gdb a.out
Reading symbols from a.out...
(gdb) break main
Breakpoint 1 at 0x4003c0: file test.c, line 19.
(gdb) commands
Type commands for breakpoint(s) 1, one per line.
End with a line saying just "end".
>bt
>end
(gdb) interpreter-exec mi "-exec-run"
^done

Breakpoint 1, main () at test.c:19
19        return foo (4);
(gdb)
...

Interestingly, the function print_frame is called twice during -exec-run:
- once during tui_on_normal_stop where the ui_out is temporarily set to
  tui->interp_ui_out (), resulting in the part after the comma in
  "Breakpoint 1, main () at test.c:19"
- once during execute_control_command, where the ui_out is the default for the
  current interpreter: mi_ui_out, which ignores calls to output text.

The commit 3a87ae656c2 "Use console uiout when executing breakpoint commands"
fixes the problem by temporarily switching to the ui_out of INTERP_CONSOLE in
execute_control_command.

This however caused a regression in redirection (escaping '#' using '\' for
git commit message convenience):
...
$ rm -f gdb.txt; gdb a.out
Reading symbols from a.out...
(gdb) break main
Breakpoint 1 at 0x4003c0: file test.c, line 19.
(gdb) commands
Type commands for breakpoint(s) 1, one per line.
End with a line saying just "end".
>bt
>end
(gdb) set logging redirect on
(gdb) set logging on
Redirecting output to gdb.txt.
Copying debug output to gdb.txt.
(gdb) run
\#0  main () at test.c:19
(gdb) q
A debugging session is active.

        Inferior 1 [process 22428] will be killed.

Quit anyway? (y or n) y
$ cat gdb.txt
Starting program: /data/gdb_versions/devel/a.out

Breakpoint 1, main () at test.c:19
19        return foo (4);
...

The problem is that the '#0  main () at test.c:19' ends up in the gdb output
output rather than in gdb.txt.  This is due to the fact that the redirect is
setup for the current ui_out (which is tui->interp_ui_out ()), while the
backtrace output is printed to the INTERP_CONSOLE ui_out.

Fix this by limiting switching to INTERP_CONSOLE ui_out to when INTERP_MI is
active.

Tested on x86_64-linux.

gdb/ChangeLog:

2019-11-21  Tom de Vries  <tdevries@suse.de>

	PR gdb/24956
	* cli/cli-script.c (execute_control_command): Only switch to
	INTERP_CONSOLE's ui_out when INTERP_MI is active.

gdb/testsuite/ChangeLog:

2019-11-21  Tom de Vries  <tdevries@suse.de>

	PR gdb/24956
	* gdb.base/ui-redirect.exp: Test output of user-defined command.

Change-Id: Id1771e7fcc9496a7d97ec2b2ea6b1487596f1ef7