This patch tunes class best_match's cutoff for rejecting meaningless
spelling suggestions.

Previously, we allowed an edit distance of up to half of the length of the
longer of the goal string and closest candidate strings, rounded down.

With this patch, we now allow only up to a third - with some tuning of
rounding (and for very short strings), to ensure that:
(a) everything that worked before still works (with the removal of a
couple of cases that shouldn't), and that
(b) the new threshold is always at least as conservative as the old
threshold and thus shouldn't offer new nonsensical suggestions (with
the possible exception of cases where transposition has helped; see
r261521 aka Damerau-Levenshtein; PR other/69968).

In particular, all of the bogus suggestions from PR c/82967 are now
no longer offered.

gcc/ChangeLog:
	PR c/82967
	* spellcheck.c (get_edit_distance_cutoff): New function.
	(selftest::test_edit_distance_unit_test_oneway): Rename to...
	(selftest::test_get_edit_distance_one_way): ...this.
	(selftest::test_get_edit_distance_unit): Rename to...
	(selftest::test_get_edit_distance_both_ways): ...this.
	(selftest::test_edit_distances): Move tests to this new function,
	and test some more pairs of strings.  Update for above renaming.
	(selftest::get_old_cutoff): New function.
	(selftest::test_get_edit_distance_cutoff): New function.
	(selftest::assert_suggested_for): New function.
	(ASSERT_SUGGESTED_FOR): New macro.
	(selftest::assert_not_suggested_for): New function.
	(ASSERT_NOT_SUGGESTED_FOR): New macro.
	(selftest::test_suggestions): New function.
	(selftest::spellcheck_c_tests): Move test_get_edit_distance_unit
	tests to selftest::test_edit_distances and call it.  Add calls to
	selftest::test_get_edit_distance_cutoff and
	selftest::test_suggestions.
	* spellcheck.h (get_edit_distance_cutoff): New function declaration.
	(best_match::consider): Replace hard-coded cutoff calculation with
	a call to...
	(best_match::get_cutoff): New declaration.
	(best_match::get_best_meaningful_candidate): Likewise.

gcc/testsuite/ChangeLog:
	PR c/82967
	* c-c++-common/attributes-1.c: Remove bogus suggestion from
	dg-prune-output.
	* gcc.dg/diagnostic-token-ranges.c (undeclared_identifier): Remove
	bogus suggestion.
	* gcc.dg/spellcheck-identifiers-4.c: New test.

From-SVN: r264335