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/cpp20_spaceship_operator.i

@@ -0,0 +1,28 @@
+%module cpp20_spaceship_operator
+
+%rename(spaceship) operator<=>;
+
+%inline %{
+#include <compare>
+
+int v = (-1 <=> 1 > 0) ? 7 : 42;
+
+// We use !(a >= b) here due to limited support for (a < b) in SWIG's parser.
+#define ALIEN !(0 <=> 1 >= 0)
+
+const int SPACE = 3 <=> 3 == 0;
+
+struct A {
+  int v;
+
+  explicit A(int v_) : v(v_) { }
+};
+
+int operator<=>(const A& a, const A& b) {
+  return a.v - b.v;
+}
+
+int f(int v = (-1 <=> 1 > 0) ? 7 : 42) { return v; }
+%}
+
+%constant int COMET = (4 <=> 2 > 0);