Use bool for is_exec_point

Use bool for is_exec_point

This changes tui_source_element::is_exec_point to be a bool.  I looked
at also changing "has_break", but it turns out that this field is used
inconsistently (sometimes as flags and sometimes as a bool), and so
needs more invesstigation before it can be changed.

gdb/ChangeLog
2019-06-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.c (tui_clear_source_content)
	(tui_source_window_base::set_is_exec_point_at): Update.
	* tui/tui-source.c (tui_set_source_content_nil): Update.
	* tui/tui-data.h (struct tui_source_element) <is_exec_point>: Now
	a bool.
	* tui/tui-data.c (init_content_element): Update.