swift234

Implement the first half of SE-0109: Remove the Boolean protocol

This removes conformance of DarwinBool and ObjCBool to the Boolean protocol,
and makes the &&/||/! operators be concrete w.r.t. Bool instead of abstract
on Boolean.

This fixes some outstanding bugs w.r.t diagnostics, but exposes some cases
where an existing diagnostic is not great.  I'll fix that in a later patch
(tracked by rdar://27391581).