Enable -Wsuggest-override

Enable -Wsuggest-override

I noticed the existence of -Wsuggest-override and so this patch
enables it for gdb.  It found a few spots that could use "override".
Also I went ahead and removed all uses of the "OVERRIDE" macro.

Using override is beneficial because it makes it harder to change a
base class and then forget to change a derived class.

Tested by the buildbot.

ChangeLog
2018-04-27  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* warning.m4 (AM_GDB_WARNINGS): Add -Wsuggest-override.
	* dwarf2loc.c (class dwarf_evaluate_loc_desc): Use "override", not
	"OVERRIDE".
	(class symbol_needs_eval_context): Likewise.
	* dwarf2read.c (mock_mapped_index::symbol_name_count)
	(mock_mapped_index::symbol_name_at): Use "override".  Remove
	"virtual".
	* dwarf2-frame.c (dwarf_expr_executor::get_addr_index): Use
	"override".
	(class dwarf_expr_executor): Use "override", not "OVERRIDE".
	* aarch64-tdep.c (instruction_reader::read): Use "override".
	(instruction_reader_test::read): Likewise.
	* arm-tdep.c (instruction_reader::read): Use "override".
	(instruction_reader_thumb::read): Likewise.

gdbserver/ChangeLog
2018-04-27  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.