Simplify psym_map_matching_symbols

Simplify psym_map_matching_symbols

This introduces a new helper function,
iterate_over_symbols_terminated, and changes psym_map_matching_symbols
to use it.  A subsequent patch will introduce a new user of this
function in the DWARF reader.

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

	* psymtab.c (map_block): Remove.
	(psym_map_matching_symbols): Use iterate_over_symbols_terminated.
	* symtab.c (iterate_over_symbols_terminated): New function.
	* symtab.c (iterate_over_symbols_terminated): Declare.