Let expand_symtabs_matching short-circuit

Let expand_symtabs_matching short-circuit

This changes expand_symtabs_exp_notify_ftype to return bool, and
updates all the uses.  Now, if the notification function returns
false, the call is short-circuited and stops examining symtabs.  This
is a step toward replacing map_symtabs_matching_filename with
expand_symtabs_matching.

gdb/ChangeLog
2021-04-17  Tom Tromey  <tom@tromey.com>

	* symtab.c (default_collect_symbol_completion_matches_break_on):
	Update.
	* symfile.h (expand_symtabs_matching): Return bool.
	* symfile.c (expand_symtabs_matching): Return bool.
	* symfile-debug.c (objfile::expand_symtabs_matching): Return
	bool.
	* quick-symbol.h (expand_symtabs_exp_notify_ftype): Return bool.
	(struct quick_symbol_functions) <expand_symtabs_matching>: Return
	bool.
	* psymtab.c (psymbol_functions::expand_symtabs_matching): Return
	bool.
	* psympriv.h (struct psymbol_functions)
	<expand_symtabs_matching>: Return bool.
	* objfiles.h (struct objfile) <expand_symtabs_matching>: Return
	bool.
	* dwarf2/read.c (struct dwarf2_gdb_index)
	<expand_symtabs_matching>: Return bool.
	(struct dwarf2_debug_names_index) <expand_symtabs_matching>:
	Return bool.
	(dw2_expand_symtabs_matching_symbol): Return bool.
	(dw2_expand_symtabs_matching_one, dw2_expand_marked_cus)
	(dw2_expand_symtabs_matching)
	(dwarf2_gdb_index::expand_symtabs_matching)
	(dwarf2_debug_names_index::expand_symtabs_matching)
	(dwarf2_debug_names_index::expand_symtabs_matching): Return bool.