Fix spaceship operator bugs, document, add tests

Remove some erroneously added brackets_increment() calls.

Reject <=> in preprocessor expressions with a clear error message (it
seems it isn't supported here - clang and gcc don't at least).

The type returned by `<=>` is not `bool`.  We pretend it's
`int` for now, which should work for how it's likely to be used
in constant expressions.

Fixes #1622

Examples/test-suite/php/cpp20_spaceship_operator_runme.php

@@ -0,0 +1,23 @@
+<?php
+
+require "tests.php";
+
+check::functions(array('f', 'spaceship'));
+check::classes(array('cpp20_spaceship_operator','A'));
+check::globals(array('v', 'SPACE'));
+
+//check::equal(ALIEN, true);
+check::equal(SPACE_get(), 1);
+check::equal(COMET, 1);
+check::equal(v_get(), 42);
+
+$x = new A(1);
+$y = new A(2);
+
+check::equal(spaceship($x, $y) < 0, true);
+check::equal(spaceship($x, $x), 0);
+check::equal(spaceship($y, $x) > 0, true);
+
+check::equal(f(), 42);
+
+check::done();