This helps with prototype inference and avoids some 'must be known
in this context' errors.

(With the fix for #2185 included.)

Issue #2111

When these are idents they are always shadowed by the boolean
constants.

This reverts commit 6e909e38.

This reverts commit 06afe21c.

Only build through librustsyntax, which some tests do rely on.

I had to xfail one existing test case (class-implements-int) because,
I think, of the same bug described in #2272.

    enter_opt was handling the (*) case wrong and causing a bounds check
    failure. Fixed it (the test case is one of the extracted ones from
    the reference manual)