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
Olly Betts