Fix TUI use of "has_break" field

Fix TUI use of "has_break" field

The TUI uses the "has_break" in two different ways: sometimes as a
boolean, and sometimes as flags.

This patch changes the TUI to be more type-safe here, and fixes the
code.  I could not find a bug that this caused, so apparently this is
just cosmetic.

This deletes some code from tui_set_disassem_content.  Whenver this is
called, I believe the TUI updates the breakpoint information
afterward, so this assignment is redundant; which is good because it
is also incorrect.

gdb/ChangeLog
2019-07-04  Tom Tromey  <tom@tromey.com>

	PR tui/24724:
	* tui/tui-winsource.c (tui_clear_source_content): Update.
	(tui_source_window_base::set_is_exec_point_at): Fix comment.
	(tui_update_breakpoint_info): Update.
	(tui_set_exec_info_content): Update.
	* tui/tui-source.c (tui_set_source_content_nil): Update.
	* tui/tui-disasm.c (tui_set_disassem_content): Don't set
	has_break.
	* tui/tui-data.h (enum tui_bp_flag): New.
	(tui_bp_flags): New enum flags type.
	(struct tui_source_element) <break_mode>: Change type.  Rename
	from has_break.
	(TUI_BP_ENABLED, TUI_BP_DISABLED, TUI_BP_HIT)
	(TUI_BP_CONDITIONAL, TUI_BP_HARDWARE): Don't define.  Now enum
	constants.
	* tui/tui-winsource.h: Fix comment.