Introduce tui_source_window_base::location_matches_p method

Introduce tui_source_window_base::location_matches_p method

This introduces the location_matches_p method, removing a spot that
explicitly examines a window's type.

gdb/ChangeLog
2019-07-17  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.c (tui_update_breakpoint_info): Use
	location_matches_p.
	* tui/tui-source.c (tui_source_window::location_matches_p): New
	method.
	* tui/tui-disasm.c (tui_disasm_window::location_matches_p): New
	method.
	* tui/tui-data.h (struct tui_source_window_base)
	<location_matches_p>: New method.
	(struct tui_source_window, struct tui_disasm_window)
	<location_matches_p>: Likewise.