Change tui_source_element::line to have type std::string

Change tui_source_element::line to have type std::string

This changes tui_source_element::line to be of type std::string.  This
reduces the number of copies made.

gdb/ChangeLog
2019-11-05  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.h (struct tui_source_element) <line>: Now a
	std::string.
	* tui/tui-winsource.c (tui_show_source_line): Update.
	* tui/tui-source.c (tui_source_window::set_contents): Update.
	* tui/tui-disasm.c (tui_disasm_window::set_contents): Update.

Change-Id: Id600f3e1d386a2911f187366e05e2ec599068dd2