Change iterate_over_symbols to return bool

Change iterate_over_symbols to return bool

This changes iterate_over_symbols to return a bool.  This allows it to
be reused in another context in a subsequent patch.

gdb/ChangeLog
2019-09-10  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_iterate_over_symbols): Return bool.
	* language.h (struct language_defn) <la_iterate_over_symbols>:
	Return bool.
	* symtab.c (iterate_over_symbols): Return bool.
	* symtab.h (iterate_over_symbols): Return bool.