Constify command_line_input

Constify command_line_input

This changes command_line_input to return a "const char *", which is
appropriate because the memory is owned by command_line_input.  Then
it fixes up the users.

I looked at making command_line_input transfer ownership to its caller
instead, but this is complicated due to the way read_next_line is
called, so I decided against it.

Tested by rebuilding.

gdb/ChangeLog
2019-11-08  Tom Tromey  <tromey@adacore.com>

	* top.c (read_command_file): Update.
	(command_line_input): Make return type const.
	* python/py-gdb-readline.c: Update.
	* linespec.c (decode_line_2): Update.
	* defs.h (command_line_input): Make return type const.
	* cli/cli-script.c (read_next_line): Make return type const.
	* ada-lang.c (get_selections): Update.

Change-Id: I27e6c9477fd1005ab5b16e0d337e4c015b6e6248