Change tui_update_locator_fullname to take a symtab

Change tui_update_locator_fullname to take a symtab

This changes tui_update_locator_fullname to take a symtab.  This
somewhat consolidates the "??" handling.

gdb/ChangeLog
2019-12-20  Tom Tromey  <tom@tromey.com>

	* tui/tui.c (tui_show_source): Update.
	* tui/tui-winsource.c (tui_display_main): Update.
	* tui/tui-stack.h (tui_update_locator_fullname): Change parameter
	to symtab.
	* tui/tui-stack.c (tui_update_locator_fullname): Change parameter
	to symtab.
	* tui/tui-disasm.c (tui_show_disassem_and_update_source): Update.

Change-Id: Ic61749517b44ac68561d829ff81f16976b830dec