Make isearch change readline prompt in TUI

Make isearch change readline prompt in TUI

PR tui/23619 points out that isearch changes the prompt in the CLI gdb
(and in Bash) -- but not in the TUI.  This turns out to be easily
fixed by removing tui_rl_saved_prompt and instead using the prompt
that readline computes.

This is stored in rl_display_prompt, which according to git was added
in readline 6.2.

gdb/ChangeLog
2019-12-20  Tom Tromey  <tom@tromey.com>

	PR tui/23619:
	* tui/tui-io.c (tui_rl_saved_prompt): Remove.
	(tui_redisplay_readline): Use rl_display_prompt.
	(tui_prep_terminal): Update.

Change-Id: Iae97e9776a5540bbe52c73b05e4707941d9cd11a